Message ID | 20150327222350.GA22887@linux.vnet.ibm.com (mailing list archive) |
---|---|
State | Awaiting Upstream, archived |
Headers | show |
On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote: > On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote: > > On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote: > > > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) > > > > > > for (i = 0; i <= ZONE_NORMAL; i++) { > > > zone = &pgdat->node_zones[i]; > > > - if (!populated_zone(zone)) > > > + if (!populated_zone(zone) || !zone_reclaimable(zone)) > > > continue; > > > > > > pfmemalloc_reserve += min_wmark_pages(zone); > > > > Do you really want zone_reclaimable()? Or do you want something more > > direct like "zone_reclaimable_pages(zone) == 0"? > > Yeah, I guess in my testing this worked out to be the same, since > zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will > always be false. Thanks! > > Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc > reserves if node has no ZONE_NORMAL") from Mel. > > We have a system with the following topology: > > # numactl -H > available: 3 nodes (0,2-3) > node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 > 23 24 25 26 27 28 29 30 31 > node 0 size: 28273 MB > node 0 free: 27323 MB > node 2 cpus: > node 2 size: 16384 MB > node 2 free: 0 MB > node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 > node 3 size: 30533 MB > node 3 free: 13273 MB > node distances: > node 0 2 3 > 0: 10 20 20 > 2: 20 10 20 > 3: 20 20 10 > > Node 2 has no free memory, because: > # cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages > 1 > > This leads to the following zoneinfo: > > Node 2, zone DMA > pages free 0 > min 1840 > low 2300 > high 2760 > scanned 0 > spanned 262144 > present 262144 > managed 262144 > ... > all_unreclaimable: 1 Blee, this is a weird configuration. > If one then attempts to allocate some normal 16M hugepages via > > echo 37 > /proc/sys/vm/nr_hugepages > > The echo never returns and kswapd2 consumes CPU cycles. > > This is because throttle_direct_reclaim ends up calling > wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...). > pfmemalloc_watermark_ok() in turn checks all zones on the node if there > are any reserves, and if so, then indicates the watermarks are ok, by > seeing if there are sufficient free pages. > > 675becce15 added a condition already for memoryless nodes. In this case, > though, the node has memory, it is just all consumed (and not > reclaimable). Effectively, though, the result is the same on this call > to pfmemalloc_watermark_ok() and thus seems like a reasonable additional > condition. > > With this change, the afore-mentioned 16M hugepage allocation attempt > succeeds and correctly round-robins between Nodes 1 and 3. I am just wondering whether this is the right/complete fix. Don't we need a similar treatment at more places? I would expect kswapd would be looping endlessly because the zone wouldn't be balanced obviously. But I would be wrong... because pgdat_balanced is doing this: /* * A special case here: * * balance_pgdat() skips over all_unreclaimable after * DEF_PRIORITY. Effectively, it considers them balanced so * they must be considered balanced here as well! */ if (!zone_reclaimable(zone)) { balanced_pages += zone->managed_pages; continue; } and zone_reclaimable is false for you as you didn't have any zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those zones right?) and so the kswapd would be woken up easily. So it looks like a mess. There are possibly other places which rely on populated_zone or for_each_populated_zone without checking reclaimability. Are those working as expected? That being said. I am not objecting to this patch. I am just trying to wrap my head around possible issues from such a weird configuration and all the consequences. > Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com> The patch as is doesn't seem to be harmful. Reviewed-by: Michal Hocko <mhocko@suse.cz> > --- > v1 -> v2: > Check against zone_reclaimable_pages, rather zone_reclaimable, based > upon feedback from Dave Hansen. Dunno, but shouldn't we use the same thing here and in pgdat_balanced? zone_reclaimable_pages seems to be used only from zone_reclaimable(). > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 5e8eadd71bac..c627fa4c991f 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) > > for (i = 0; i <= ZONE_NORMAL; i++) { > zone = &pgdat->node_zones[i]; > - if (!populated_zone(zone)) > + if (!populated_zone(zone) || > + zone_reclaimable_pages(zone) == 0) > continue; > > pfmemalloc_reserve += min_wmark_pages(zone); >
On 03/31/2015 11:48 AM, Michal Hocko wrote: > On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote: >> On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote: >>> On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote: >>>> @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) >>>> >>>> for (i = 0; i <= ZONE_NORMAL; i++) { >>>> zone = &pgdat->node_zones[i]; >>>> - if (!populated_zone(zone)) >>>> + if (!populated_zone(zone) || !zone_reclaimable(zone)) >>>> continue; >>>> >>>> pfmemalloc_reserve += min_wmark_pages(zone); >>> >>> Do you really want zone_reclaimable()? Or do you want something more >>> direct like "zone_reclaimable_pages(zone) == 0"? >> >> Yeah, I guess in my testing this worked out to be the same, since >> zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will >> always be false. Thanks! >> >> Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc >> reserves if node has no ZONE_NORMAL") from Mel. >> >> We have a system with the following topology: >> >> # numactl -H >> available: 3 nodes (0,2-3) >> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 >> 23 24 25 26 27 28 29 30 31 >> node 0 size: 28273 MB >> node 0 free: 27323 MB >> node 2 cpus: >> node 2 size: 16384 MB >> node 2 free: 0 MB >> node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 >> node 3 size: 30533 MB >> node 3 free: 13273 MB >> node distances: >> node 0 2 3 >> 0: 10 20 20 >> 2: 20 10 20 >> 3: 20 20 10 >> >> Node 2 has no free memory, because: >> # cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages >> 1 >> >> This leads to the following zoneinfo: >> >> Node 2, zone DMA >> pages free 0 >> min 1840 >> low 2300 >> high 2760 >> scanned 0 >> spanned 262144 >> present 262144 >> managed 262144 >> ... >> all_unreclaimable: 1 > > Blee, this is a weird configuration. > >> If one then attempts to allocate some normal 16M hugepages via >> >> echo 37 > /proc/sys/vm/nr_hugepages >> >> The echo never returns and kswapd2 consumes CPU cycles. >> >> This is because throttle_direct_reclaim ends up calling >> wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...). >> pfmemalloc_watermark_ok() in turn checks all zones on the node if there >> are any reserves, and if so, then indicates the watermarks are ok, by >> seeing if there are sufficient free pages. >> >> 675becce15 added a condition already for memoryless nodes. In this case, >> though, the node has memory, it is just all consumed (and not >> reclaimable). Effectively, though, the result is the same on this call >> to pfmemalloc_watermark_ok() and thus seems like a reasonable additional >> condition. >> >> With this change, the afore-mentioned 16M hugepage allocation attempt >> succeeds and correctly round-robins between Nodes 1 and 3. > > I am just wondering whether this is the right/complete fix. Don't we > need a similar treatment at more places? > I would expect kswapd would be looping endlessly because the zone > wouldn't be balanced obviously. But I would be wrong... because > pgdat_balanced is doing this: > /* > * A special case here: > * > * balance_pgdat() skips over all_unreclaimable after > * DEF_PRIORITY. Effectively, it considers them balanced so > * they must be considered balanced here as well! > */ > if (!zone_reclaimable(zone)) { > balanced_pages += zone->managed_pages; > continue; > } > > and zone_reclaimable is false for you as you didn't have any > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those > zones right?) and so the kswapd would be woken up easily. So it looks > like a mess. Yeah, looks like a much cleaner/complete solution would be to remove such zones from zonelists. But that means covering all situations when these hugepages are allocated/removed and the approach then looks similar to memory hotplug. Also I'm not sure if the ability to actually allocate the reserved hugepage would be impossible due to not being reachable by a zonelist... > There are possibly other places which rely on populated_zone or > for_each_populated_zone without checking reclaimability. Are those > working as expected? Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point in waking it up just to let it immediately go to sleep again. > That being said. I am not objecting to this patch. I am just trying to > wrap my head around possible issues from such a weird configuration and > all the consequences. > >> Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com> > > The patch as is doesn't seem to be harmful. > > Reviewed-by: Michal Hocko <mhocko@suse.cz> > >> --- >> v1 -> v2: >> Check against zone_reclaimable_pages, rather zone_reclaimable, based >> upon feedback from Dave Hansen. > > Dunno, but shouldn't we use the same thing here and in pgdat_balanced? > zone_reclaimable_pages seems to be used only from zone_reclaimable(). pgdat_balanced() has a different goal than pfmemalloc_watermark_ok() and needs to match what balance_pgdat() does, which includes considering NR_PAGES_SCANNED through zone_reclaimable(). For the situation considered in this patch, result of zone_reclaimable() will match the test zone_reclaimable_pages(zone) == 0, so it is fine I think. What I find somewhat worrying though is that we could potentially break the pfmemalloc_watermark_ok() test in situations where zone_reclaimable_pages(zone) == 0 is a transient situation (and not a permanently allocated hugepage). In that case, the throttling is supposed to help system recover, and we might be breaking that ability with this patch, no? >> diff --git a/mm/vmscan.c b/mm/vmscan.c >> index 5e8eadd71bac..c627fa4c991f 100644 >> --- a/mm/vmscan.c >> +++ b/mm/vmscan.c >> @@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) >> >> for (i = 0; i <= ZONE_NORMAL; i++) { >> zone = &pgdat->node_zones[i]; >> - if (!populated_zone(zone)) >> + if (!populated_zone(zone) || >> + zone_reclaimable_pages(zone) == 0) >> continue; >> >> pfmemalloc_reserve += min_wmark_pages(zone); >> >
On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote: > On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote: > > On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote: > > > On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote: > > > > @@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) > > > > > > > > for (i = 0; i <= ZONE_NORMAL; i++) { > > > > zone = &pgdat->node_zones[i]; > > > > - if (!populated_zone(zone)) > > > > + if (!populated_zone(zone) || !zone_reclaimable(zone)) > > > > continue; > > > > > > > > pfmemalloc_reserve += min_wmark_pages(zone); > > > > > > Do you really want zone_reclaimable()? Or do you want something more > > > direct like "zone_reclaimable_pages(zone) == 0"? > > > > Yeah, I guess in my testing this worked out to be the same, since > > zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will > > always be false. Thanks! > > > > Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc > > reserves if node has no ZONE_NORMAL") from Mel. > > > > We have a system with the following topology: > > > > # numactl -H > > available: 3 nodes (0,2-3) > > node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 > > 23 24 25 26 27 28 29 30 31 > > node 0 size: 28273 MB > > node 0 free: 27323 MB > > node 2 cpus: > > node 2 size: 16384 MB > > node 2 free: 0 MB > > node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 > > node 3 size: 30533 MB > > node 3 free: 13273 MB > > node distances: > > node 0 2 3 > > 0: 10 20 20 > > 2: 20 10 20 > > 3: 20 20 10 > > > > Node 2 has no free memory, because: > > # cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages > > 1 > > > > This leads to the following zoneinfo: > > > > Node 2, zone DMA > > pages free 0 > > min 1840 > > low 2300 > > high 2760 > > scanned 0 > > spanned 262144 > > present 262144 > > managed 262144 > > ... > > all_unreclaimable: 1 > > Blee, this is a weird configuration. Yep, super gross. It's relatively rare in the field, thankfully. But 16G pages definitely make it pretty likely to hit (as in, I've seen it once before :) > > If one then attempts to allocate some normal 16M hugepages via > > > > echo 37 > /proc/sys/vm/nr_hugepages > > > > The echo never returns and kswapd2 consumes CPU cycles. > > > > This is because throttle_direct_reclaim ends up calling > > wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...). > > pfmemalloc_watermark_ok() in turn checks all zones on the node if there > > are any reserves, and if so, then indicates the watermarks are ok, by > > seeing if there are sufficient free pages. > > > > 675becce15 added a condition already for memoryless nodes. In this case, > > though, the node has memory, it is just all consumed (and not > > reclaimable). Effectively, though, the result is the same on this call > > to pfmemalloc_watermark_ok() and thus seems like a reasonable additional > > condition. > > > > With this change, the afore-mentioned 16M hugepage allocation attempt > > succeeds and correctly round-robins between Nodes 1 and 3. > > I am just wondering whether this is the right/complete fix. Don't we > need a similar treatment at more places? Almost certainly needs an audit. Exhausted nodes are tough to reproduce easily (fully exhausted, that is), for me. > I would expect kswapd would be looping endlessly because the zone > wouldn't be balanced obviously. But I would be wrong... because > pgdat_balanced is doing this: > /* > * A special case here: > * > * balance_pgdat() skips over all_unreclaimable after > * DEF_PRIORITY. Effectively, it considers them balanced so > * they must be considered balanced here as well! > */ > if (!zone_reclaimable(zone)) { > balanced_pages += zone->managed_pages; > continue; > } > > and zone_reclaimable is false for you as you didn't have any > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those > zones right?) and so the kswapd would be woken up easily. So it looks > like a mess. My understanding, and I could easily be wrong, is that kswapd2 (node 2 is the exhausted one) spins endlessly, because the reclaim logic sees that we are reclaiming from somewhere but the allocation request for node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will never complete, so we just continue to reclaim. > There are possibly other places which rely on populated_zone or > for_each_populated_zone without checking reclaimability. Are those > working as expected? Not yet verified, admittedly. > That being said. I am not objecting to this patch. I am just trying to > wrap my head around possible issues from such a weird configuration and > all the consequences. Yeah, there are almost certainly more. Luckily, our test organization is hammering this configuration, so hopefully I'll get reports about further issues soon, if there are any, with the patch applied. Thanks, Nish
On 03.04.2015 [09:57:35 +0200], Vlastimil Babka wrote: > On 03/31/2015 11:48 AM, Michal Hocko wrote: > >On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote: > >>On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote: > >>>On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote: > >>>>@@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) > >>>> > >>>> for (i = 0; i <= ZONE_NORMAL; i++) { > >>>> zone = &pgdat->node_zones[i]; > >>>>- if (!populated_zone(zone)) > >>>>+ if (!populated_zone(zone) || !zone_reclaimable(zone)) > >>>> continue; > >>>> > >>>> pfmemalloc_reserve += min_wmark_pages(zone); > >>> > >>>Do you really want zone_reclaimable()? Or do you want something more > >>>direct like "zone_reclaimable_pages(zone) == 0"? > >> > >>Yeah, I guess in my testing this worked out to be the same, since > >>zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will > >>always be false. Thanks! > >> > >>Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc > >>reserves if node has no ZONE_NORMAL") from Mel. > >> > >>We have a system with the following topology: > >> > >># numactl -H > >>available: 3 nodes (0,2-3) > >>node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 > >>23 24 25 26 27 28 29 30 31 > >>node 0 size: 28273 MB > >>node 0 free: 27323 MB > >>node 2 cpus: > >>node 2 size: 16384 MB > >>node 2 free: 0 MB > >>node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 > >>node 3 size: 30533 MB > >>node 3 free: 13273 MB > >>node distances: > >>node 0 2 3 > >> 0: 10 20 20 > >> 2: 20 10 20 > >> 3: 20 20 10 > >> > >>Node 2 has no free memory, because: > >># cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages > >>1 > >> > >>This leads to the following zoneinfo: > >> > >>Node 2, zone DMA > >> pages free 0 > >> min 1840 > >> low 2300 > >> high 2760 > >> scanned 0 > >> spanned 262144 > >> present 262144 > >> managed 262144 > >>... > >> all_unreclaimable: 1 > > > >Blee, this is a weird configuration. > > > >>If one then attempts to allocate some normal 16M hugepages via > >> > >>echo 37 > /proc/sys/vm/nr_hugepages > >> > >>The echo never returns and kswapd2 consumes CPU cycles. > >> > >>This is because throttle_direct_reclaim ends up calling > >>wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...). > >>pfmemalloc_watermark_ok() in turn checks all zones on the node if there > >>are any reserves, and if so, then indicates the watermarks are ok, by > >>seeing if there are sufficient free pages. > >> > >>675becce15 added a condition already for memoryless nodes. In this case, > >>though, the node has memory, it is just all consumed (and not > >>reclaimable). Effectively, though, the result is the same on this call > >>to pfmemalloc_watermark_ok() and thus seems like a reasonable additional > >>condition. > >> > >>With this change, the afore-mentioned 16M hugepage allocation attempt > >>succeeds and correctly round-robins between Nodes 1 and 3. > > > >I am just wondering whether this is the right/complete fix. Don't we > >need a similar treatment at more places? > >I would expect kswapd would be looping endlessly because the zone > >wouldn't be balanced obviously. But I would be wrong... because > >pgdat_balanced is doing this: > > /* > > * A special case here: > > * > > * balance_pgdat() skips over all_unreclaimable after > > * DEF_PRIORITY. Effectively, it considers them balanced so > > * they must be considered balanced here as well! > > */ > > if (!zone_reclaimable(zone)) { > > balanced_pages += zone->managed_pages; > > continue; > > } > > > >and zone_reclaimable is false for you as you didn't have any > >zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it > >would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those > >zones right?) and so the kswapd would be woken up easily. So it looks > >like a mess. > > Yeah, looks like a much cleaner/complete solution would be to remove > such zones from zonelists. But that means covering all situations > when these hugepages are allocated/removed and the approach then > looks similar to memory hotplug. > Also I'm not sure if the ability to actually allocate the reserved > hugepage would be impossible due to not being reachable by a > zonelist... > > >There are possibly other places which rely on populated_zone or > >for_each_populated_zone without checking reclaimability. Are those > >working as expected? > > Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point > in waking it up just to let it immediately go to sleep again. > > >That being said. I am not objecting to this patch. I am just trying to > >wrap my head around possible issues from such a weird configuration and > >all the consequences. > > > >>Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com> > > > >The patch as is doesn't seem to be harmful. > > > >Reviewed-by: Michal Hocko <mhocko@suse.cz> > > > >>--- > >>v1 -> v2: > >> Check against zone_reclaimable_pages, rather zone_reclaimable, based > >> upon feedback from Dave Hansen. > > > >Dunno, but shouldn't we use the same thing here and in pgdat_balanced? > >zone_reclaimable_pages seems to be used only from zone_reclaimable(). > > pgdat_balanced() has a different goal than pfmemalloc_watermark_ok() > and needs to match what balance_pgdat() does, which includes > considering NR_PAGES_SCANNED through zone_reclaimable(). For the > situation considered in this patch, result of zone_reclaimable() > will match the test zone_reclaimable_pages(zone) == 0, so it is fine > I think. Right. > What I find somewhat worrying though is that we could potentially > break the pfmemalloc_watermark_ok() test in situations where > zone_reclaimable_pages(zone) == 0 is a transient situation (and not > a permanently allocated hugepage). In that case, the throttling is > supposed to help system recover, and we might be breaking that > ability with this patch, no? Well, if it's transient, we'll skip it this time through, and once there are reclaimable pages, we should notice it again. I'm not familiar enough with this logic, so I'll read through the code again soon to see if your concern is valid, as best I can. Thanks, Nish
On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote: > On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote: [...] > > I would expect kswapd would be looping endlessly because the zone > > wouldn't be balanced obviously. But I would be wrong... because > > pgdat_balanced is doing this: > > /* > > * A special case here: > > * > > * balance_pgdat() skips over all_unreclaimable after > > * DEF_PRIORITY. Effectively, it considers them balanced so > > * they must be considered balanced here as well! > > */ > > if (!zone_reclaimable(zone)) { > > balanced_pages += zone->managed_pages; > > continue; > > } > > > > and zone_reclaimable is false for you as you didn't have any > > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it > > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those > > zones right?) and so the kswapd would be woken up easily. So it looks > > like a mess. > > My understanding, and I could easily be wrong, is that kswapd2 (node 2 > is the exhausted one) spins endlessly, because the reclaim logic sees > that we are reclaiming from somewhere but the allocation request for > node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will > never complete, so we just continue to reclaim. __GFP_THISNODE would be waking up kswapd2 again and again, that is true. I am just wondering whether we will have any __GFP_THISNODE allocations for a node without CPUs (numa_node_id() shouldn't return such a node AFAICS). Maybe if somebody is bound to Node2 explicitly but I would consider this as a misconfiguration.
On 03.04.2015 [20:24:45 +0200], Michal Hocko wrote: > On Fri 03-04-15 10:43:57, Nishanth Aravamudan wrote: > > On 31.03.2015 [11:48:29 +0200], Michal Hocko wrote: > [...] > > > I would expect kswapd would be looping endlessly because the zone > > > wouldn't be balanced obviously. But I would be wrong... because > > > pgdat_balanced is doing this: > > > /* > > > * A special case here: > > > * > > > * balance_pgdat() skips over all_unreclaimable after > > > * DEF_PRIORITY. Effectively, it considers them balanced so > > > * they must be considered balanced here as well! > > > */ > > > if (!zone_reclaimable(zone)) { > > > balanced_pages += zone->managed_pages; > > > continue; > > > } > > > > > > and zone_reclaimable is false for you as you didn't have any > > > zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it > > > would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those > > > zones right?) and so the kswapd would be woken up easily. So it looks > > > like a mess. > > > > My understanding, and I could easily be wrong, is that kswapd2 (node 2 > > is the exhausted one) spins endlessly, because the reclaim logic sees > > that we are reclaiming from somewhere but the allocation request for > > node 2 (which is __GFP_THISNODE for hugepages, not GFP_THISNODE) will > > never complete, so we just continue to reclaim. > > __GFP_THISNODE would be waking up kswapd2 again and again, that is true. Right, one idea I had for this was ensuring that we perform reclaim with somehow some knowledge of __GFP_THISNODE -- that is it needs to be somewhat targetted in order to actually help satisfy the current allocation. But it got pretty hairy fast and I didn't want to break the world :) > I am just wondering whether we will have any __GFP_THISNODE allocations > for a node without CPUs (numa_node_id() shouldn't return such a node > AFAICS). Maybe if somebody is bound to Node2 explicitly but I would > consider this as a misconfiguration. Right, I'd need to check what happens if in our setup you taskset to node2 and tried to force memory to be local -- I think you'd either be killed immediately, or the kernel will just disagree with your binding since it's invalid (e.g., that will happen if you try to bind to a memoryless node, I think). Keep in mind that although in my config node2 had no CPUs, that's not a hard & fast requirement. I do believe in a previous iteration of this bug, the exhausted node had no free memory but did have cpus assigned to it. -Nish
On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote: > On 03.04.2015 [09:57:35 +0200], Vlastimil Babka wrote: > > On 03/31/2015 11:48 AM, Michal Hocko wrote: > > >On Fri 27-03-15 15:23:50, Nishanth Aravamudan wrote: > > >>On 27.03.2015 [13:17:59 -0700], Dave Hansen wrote: > > >>>On 03/27/2015 12:28 PM, Nishanth Aravamudan wrote: > > >>>>@@ -2585,7 +2585,7 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) > > >>>> > > >>>> for (i = 0; i <= ZONE_NORMAL; i++) { > > >>>> zone = &pgdat->node_zones[i]; > > >>>>- if (!populated_zone(zone)) > > >>>>+ if (!populated_zone(zone) || !zone_reclaimable(zone)) > > >>>> continue; > > >>>> > > >>>> pfmemalloc_reserve += min_wmark_pages(zone); > > >>> > > >>>Do you really want zone_reclaimable()? Or do you want something more > > >>>direct like "zone_reclaimable_pages(zone) == 0"? > > >> > > >>Yeah, I guess in my testing this worked out to be the same, since > > >>zone_reclaimable_pages(zone) is 0 and so zone_reclaimable(zone) will > > >>always be false. Thanks! > > >> > > >>Based upon 675becce15 ("mm: vmscan: do not throttle based on pfmemalloc > > >>reserves if node has no ZONE_NORMAL") from Mel. > > >> > > >>We have a system with the following topology: > > >> > > >># numactl -H > > >>available: 3 nodes (0,2-3) > > >>node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 > > >>23 24 25 26 27 28 29 30 31 > > >>node 0 size: 28273 MB > > >>node 0 free: 27323 MB > > >>node 2 cpus: > > >>node 2 size: 16384 MB > > >>node 2 free: 0 MB > > >>node 3 cpus: 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 > > >>node 3 size: 30533 MB > > >>node 3 free: 13273 MB > > >>node distances: > > >>node 0 2 3 > > >> 0: 10 20 20 > > >> 2: 20 10 20 > > >> 3: 20 20 10 > > >> > > >>Node 2 has no free memory, because: > > >># cat /sys/devices/system/node/node2/hugepages/hugepages-16777216kB/nr_hugepages > > >>1 > > >> > > >>This leads to the following zoneinfo: > > >> > > >>Node 2, zone DMA > > >> pages free 0 > > >> min 1840 > > >> low 2300 > > >> high 2760 > > >> scanned 0 > > >> spanned 262144 > > >> present 262144 > > >> managed 262144 > > >>... > > >> all_unreclaimable: 1 > > > > > >Blee, this is a weird configuration. > > > > > >>If one then attempts to allocate some normal 16M hugepages via > > >> > > >>echo 37 > /proc/sys/vm/nr_hugepages > > >> > > >>The echo never returns and kswapd2 consumes CPU cycles. > > >> > > >>This is because throttle_direct_reclaim ends up calling > > >>wait_event(pfmemalloc_wait, pfmemalloc_watermark_ok...). > > >>pfmemalloc_watermark_ok() in turn checks all zones on the node if there > > >>are any reserves, and if so, then indicates the watermarks are ok, by > > >>seeing if there are sufficient free pages. > > >> > > >>675becce15 added a condition already for memoryless nodes. In this case, > > >>though, the node has memory, it is just all consumed (and not > > >>reclaimable). Effectively, though, the result is the same on this call > > >>to pfmemalloc_watermark_ok() and thus seems like a reasonable additional > > >>condition. > > >> > > >>With this change, the afore-mentioned 16M hugepage allocation attempt > > >>succeeds and correctly round-robins between Nodes 1 and 3. > > > > > >I am just wondering whether this is the right/complete fix. Don't we > > >need a similar treatment at more places? > > >I would expect kswapd would be looping endlessly because the zone > > >wouldn't be balanced obviously. But I would be wrong... because > > >pgdat_balanced is doing this: > > > /* > > > * A special case here: > > > * > > > * balance_pgdat() skips over all_unreclaimable after > > > * DEF_PRIORITY. Effectively, it considers them balanced so > > > * they must be considered balanced here as well! > > > */ > > > if (!zone_reclaimable(zone)) { > > > balanced_pages += zone->managed_pages; > > > continue; > > > } > > > > > >and zone_reclaimable is false for you as you didn't have any > > >zone_reclaimable_pages(). But wakeup_kswapd doesn't do this check so it > > >would see !zone_balanced() AFAICS (build_zonelists doesn't ignore those > > >zones right?) and so the kswapd would be woken up easily. So it looks > > >like a mess. > > > > Yeah, looks like a much cleaner/complete solution would be to remove > > such zones from zonelists. But that means covering all situations > > when these hugepages are allocated/removed and the approach then > > looks similar to memory hotplug. > > Also I'm not sure if the ability to actually allocate the reserved > > hugepage would be impossible due to not being reachable by a > > zonelist... > > > > >There are possibly other places which rely on populated_zone or > > >for_each_populated_zone without checking reclaimability. Are those > > >working as expected? > > > > Yeah. At least the wakeup_kswapd case should be fixed IMHO. No point > > in waking it up just to let it immediately go to sleep again. > > > > >That being said. I am not objecting to this patch. I am just trying to > > >wrap my head around possible issues from such a weird configuration and > > >all the consequences. > > > > > >>Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com> > > > > > >The patch as is doesn't seem to be harmful. > > > > > >Reviewed-by: Michal Hocko <mhocko@suse.cz> > > > > > >>--- > > >>v1 -> v2: > > >> Check against zone_reclaimable_pages, rather zone_reclaimable, based > > >> upon feedback from Dave Hansen. > > > > > >Dunno, but shouldn't we use the same thing here and in pgdat_balanced? > > >zone_reclaimable_pages seems to be used only from zone_reclaimable(). > > > > pgdat_balanced() has a different goal than pfmemalloc_watermark_ok() > > and needs to match what balance_pgdat() does, which includes > > considering NR_PAGES_SCANNED through zone_reclaimable(). For the > > situation considered in this patch, result of zone_reclaimable() > > will match the test zone_reclaimable_pages(zone) == 0, so it is fine > > I think. > > Right. > > > What I find somewhat worrying though is that we could potentially > > break the pfmemalloc_watermark_ok() test in situations where > > zone_reclaimable_pages(zone) == 0 is a transient situation (and not > > a permanently allocated hugepage). In that case, the throttling is > > supposed to help system recover, and we might be breaking that > > ability with this patch, no? > > Well, if it's transient, we'll skip it this time through, and once there > are reclaimable pages, we should notice it again. > > I'm not familiar enough with this logic, so I'll read through the code > again soon to see if your concern is valid, as best I can. In reviewing the code, I think that transiently unreclaimable zones will lead to some higher direct reclaim rates and possible contention, but shouldn't cause any major harm. The likelihood of that situation, as well, in a non-reserved memory setup like the one I described, seems exceedingly low. Thanks, Nish
On 05/06/2015 12:09 AM, Nishanth Aravamudan wrote: > On 03.04.2015 [10:45:56 -0700], Nishanth Aravamudan wrote: >>> What I find somewhat worrying though is that we could potentially >>> break the pfmemalloc_watermark_ok() test in situations where >>> zone_reclaimable_pages(zone) == 0 is a transient situation (and not >>> a permanently allocated hugepage). In that case, the throttling is >>> supposed to help system recover, and we might be breaking that >>> ability with this patch, no? >> >> Well, if it's transient, we'll skip it this time through, and once there >> are reclaimable pages, we should notice it again. >> >> I'm not familiar enough with this logic, so I'll read through the code >> again soon to see if your concern is valid, as best I can. > > In reviewing the code, I think that transiently unreclaimable zones will > lead to some higher direct reclaim rates and possible contention, but > shouldn't cause any major harm. The likelihood of that situation, as > well, in a non-reserved memory setup like the one I described, seems > exceedingly low. OK, I guess when a reasonably configured system has nothing to reclaim, it's already busted and throttling won't change much. Consider the patch Acked-by: Vlastimil Babka <vbabka@suse.cz> > Thanks, > Nish >
diff --git a/mm/vmscan.c b/mm/vmscan.c index 5e8eadd71bac..c627fa4c991f 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2646,7 +2646,8 @@ static bool pfmemalloc_watermark_ok(pg_data_t *pgdat) for (i = 0; i <= ZONE_NORMAL; i++) { zone = &pgdat->node_zones[i]; - if (!populated_zone(zone)) + if (!populated_zone(zone) || + zone_reclaimable_pages(zone) == 0) continue; pfmemalloc_reserve += min_wmark_pages(zone);