| Message ID | 20230807110936.21819-47-zhengqi.arch@bytedance.com |
|---|---|
| State | New |
| Headers | show |
| Series | use refcount+RCU method to implement lockless slab shrink | expand |
On Mon, Aug 07, 2023 at 07:09:34PM +0800, Qi Zheng wrote: > Like global slab shrink, this commit also uses refcount+RCU method to make > memcg slab shrink lockless. This patch does random code cleanups amongst the actual RCU changes. Can you please move the cleanups to a spearate patch to reduce the noise in this one? > diff --git a/mm/shrinker.c b/mm/shrinker.c > index d318f5621862..fee6f62904fb 100644 > --- a/mm/shrinker.c > +++ b/mm/shrinker.c > @@ -107,6 +107,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg, > lockdep_is_held(&shrinker_rwsem)); > } > > +static struct shrinker_info *shrinker_info_rcu(struct mem_cgroup *memcg, > + int nid) > +{ > + return rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); > +} This helper doesn't add value. It doesn't tell me that rcu_read_lock() needs to be held when it is called, for one.... > static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_size, > int old_size, int new_nr_max) > { > @@ -198,7 +204,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) > struct shrinker_info_unit *unit; > > rcu_read_lock(); > - info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); > + info = shrinker_info_rcu(memcg, nid); ... whilst the original code here was obviously correct. > unit = info->unit[shriner_id_to_index(shrinker_id)]; > if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) { > /* Pairs with smp mb in shrink_slab() */ > @@ -211,7 +217,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) > > static DEFINE_IDR(shrinker_idr); > > -static int prealloc_memcg_shrinker(struct shrinker *shrinker) > +static int shrinker_memcg_alloc(struct shrinker *shrinker) Cleanups in a separate patch. > @@ -253,10 +258,15 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker, > { > struct shrinker_info *info; > struct shrinker_info_unit *unit; > + long nr_deferred; > > - info = shrinker_info_protected(memcg, nid); > + rcu_read_lock(); > + info = shrinker_info_rcu(memcg, nid); > unit = info->unit[shriner_id_to_index(shrinker->id)]; > - return atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0); > + nr_deferred = atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0); > + rcu_read_unlock(); > + > + return nr_deferred; > } This adds two rcu_read_lock() sections to every call to do_shrink_slab(). It's not at all clear ifrom any of the other code that do_shrink_slab() now has internal rcu_read_lock() sections.... > @@ -464,18 +480,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, > if (!mem_cgroup_online(memcg)) > return 0; > > - if (!down_read_trylock(&shrinker_rwsem)) > - return 0; > - > - info = shrinker_info_protected(memcg, nid); > +again: > + rcu_read_lock(); > + info = shrinker_info_rcu(memcg, nid); > if (unlikely(!info)) > goto unlock; > > - for (; index < shriner_id_to_index(info->map_nr_max); index++) { > + if (index < shriner_id_to_index(info->map_nr_max)) { > struct shrinker_info_unit *unit; > > unit = info->unit[index]; > > + /* > + * The shrinker_info_unit will not be freed, so we can > + * safely release the RCU lock here. > + */ > + rcu_read_unlock(); Why - what guarantees that the shrinker_info_unit exists at this point? We hold no reference to it, we hold no reference to any shrinker, etc. What provides this existence guarantee? > + > for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) { > struct shrink_control sc = { > .gfp_mask = gfp_mask, > @@ -485,12 +506,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, > struct shrinker *shrinker; > int shrinker_id = calc_shrinker_id(index, offset); > > + rcu_read_lock(); > shrinker = idr_find(&shrinker_idr, shrinker_id); > - if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) { > - if (!shrinker) > - clear_bit(offset, unit->map); > + if (unlikely(!shrinker || !shrinker_try_get(shrinker))) { > + clear_bit(offset, unit->map); > + rcu_read_unlock(); > continue; > } > + rcu_read_unlock(); > > /* Call non-slab shrinkers even though kmem is disabled */ > if (!memcg_kmem_online() && > @@ -523,15 +546,20 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, > set_shrinker_bit(memcg, nid, shrinker_id); > } > freed += ret; > - > - if (rwsem_is_contended(&shrinker_rwsem)) { > - freed = freed ? : 1; > - goto unlock; > - } > + shrinker_put(shrinker); Ok, so why is this safe to call without holding the rcu read lock? The global shrinker has to hold the rcu_read_lock() whilst calling shrinker_put() to guarantee the validity of the list next pointer, but we don't hold off RCU here so what guarantees a racing global shrinker walk doesn't trip over this shrinker_put() call dropping the refcount to zero and freeing occuring in a different context... > + /* > + * We have already exited the read-side of rcu critical section > + * before calling do_shrink_slab(), the shrinker_info may be > + * released in expand_one_shrinker_info(), so reacquire the > + * shrinker_info. > + */ > + index++; > + goto again; With that, what makes the use of shrinker_info in xchg_nr_deferred_memcg() in do_shrink_slab() coherent and valid? -Dave.
Hi Dave, On 2023/8/8 10:44, Dave Chinner wrote: > On Mon, Aug 07, 2023 at 07:09:34PM +0800, Qi Zheng wrote: >> Like global slab shrink, this commit also uses refcount+RCU method to make >> memcg slab shrink lockless. > > This patch does random code cleanups amongst the actual RCU changes. > Can you please move the cleanups to a spearate patch to reduce the > noise in this one? Sure, will do. > >> diff --git a/mm/shrinker.c b/mm/shrinker.c >> index d318f5621862..fee6f62904fb 100644 >> --- a/mm/shrinker.c >> +++ b/mm/shrinker.c >> @@ -107,6 +107,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg, >> lockdep_is_held(&shrinker_rwsem)); >> } >> >> +static struct shrinker_info *shrinker_info_rcu(struct mem_cgroup *memcg, >> + int nid) >> +{ >> + return rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); >> +} > > This helper doesn't add value. It doesn't tell me that > rcu_read_lock() needs to be held when it is called, for one.... How about adding a comment or an assertion here? > >> static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_size, >> int old_size, int new_nr_max) >> { >> @@ -198,7 +204,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) >> struct shrinker_info_unit *unit; >> >> rcu_read_lock(); >> - info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); >> + info = shrinker_info_rcu(memcg, nid); > > ... whilst the original code here was obviously correct. > >> unit = info->unit[shriner_id_to_index(shrinker_id)]; >> if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) { >> /* Pairs with smp mb in shrink_slab() */ >> @@ -211,7 +217,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) >> >> static DEFINE_IDR(shrinker_idr); >> >> -static int prealloc_memcg_shrinker(struct shrinker *shrinker) >> +static int shrinker_memcg_alloc(struct shrinker *shrinker) > > Cleanups in a separate patch. OK. > >> @@ -253,10 +258,15 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker, >> { >> struct shrinker_info *info; >> struct shrinker_info_unit *unit; >> + long nr_deferred; >> >> - info = shrinker_info_protected(memcg, nid); >> + rcu_read_lock(); >> + info = shrinker_info_rcu(memcg, nid); >> unit = info->unit[shriner_id_to_index(shrinker->id)]; >> - return atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0); >> + nr_deferred = atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0); >> + rcu_read_unlock(); >> + >> + return nr_deferred; >> } > > This adds two rcu_read_lock() sections to every call to > do_shrink_slab(). It's not at all clear ifrom any of the other code > that do_shrink_slab() now has internal rcu_read_lock() sections.... The xchg_nr_deferred_memcg() will only be called in shrink_slab_memcg(), so other code doesn't need to know that information? > >> @@ -464,18 +480,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, >> if (!mem_cgroup_online(memcg)) >> return 0; >> >> - if (!down_read_trylock(&shrinker_rwsem)) >> - return 0; >> - >> - info = shrinker_info_protected(memcg, nid); >> +again: >> + rcu_read_lock(); >> + info = shrinker_info_rcu(memcg, nid); >> if (unlikely(!info)) >> goto unlock; >> >> - for (; index < shriner_id_to_index(info->map_nr_max); index++) { >> + if (index < shriner_id_to_index(info->map_nr_max)) { >> struct shrinker_info_unit *unit; >> >> unit = info->unit[index]; >> >> + /* >> + * The shrinker_info_unit will not be freed, so we can >> + * safely release the RCU lock here. >> + */ >> + rcu_read_unlock(); > > Why - what guarantees that the shrinker_info_unit exists at this > point? We hold no reference to it, we hold no reference to any > shrinker, etc. What provides this existence guarantee? The shrinker_info_unit is never freed unless the memcg is destroyed. Here we hold the refcount of this memcg (mem_cgroup_iter() --> css_tryget()), so the shrinker_info_unit will not be freed. > >> + >> for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) { >> struct shrink_control sc = { >> .gfp_mask = gfp_mask, >> @@ -485,12 +506,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, >> struct shrinker *shrinker; >> int shrinker_id = calc_shrinker_id(index, offset); >> >> + rcu_read_lock(); >> shrinker = idr_find(&shrinker_idr, shrinker_id); >> - if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) { >> - if (!shrinker) >> - clear_bit(offset, unit->map); >> + if (unlikely(!shrinker || !shrinker_try_get(shrinker))) { >> + clear_bit(offset, unit->map); >> + rcu_read_unlock(); >> continue; >> } >> + rcu_read_unlock(); >> >> /* Call non-slab shrinkers even though kmem is disabled */ >> if (!memcg_kmem_online() && >> @@ -523,15 +546,20 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, >> set_shrinker_bit(memcg, nid, shrinker_id); >> } >> freed += ret; >> - >> - if (rwsem_is_contended(&shrinker_rwsem)) { >> - freed = freed ? : 1; >> - goto unlock; >> - } >> + shrinker_put(shrinker); > > Ok, so why is this safe to call without holding the rcu read lock? > The global shrinker has to hold the rcu_read_lock() whilst calling > shrinker_put() to guarantee the validity of the list next pointer, > but we don't hold off RCU here so what guarantees a racing global > shrinker walk doesn't trip over this shrinker_put() call dropping > the refcount to zero and freeing occuring in a different context... This will not be a problem, even if shrinker::refcount is reduced to 0 here, the racing global shrinker walk already holds the rcu lock. shrink_slab shrink_slab_memcg =========== ================= rcu_read_lock() shrinker_put() shrinker_put() And in shrink_slab_memcg(), the shrinker is not required to traverse the next bit in the shrinker_info_unit::map, so there is no need to hold the rcu lock to ensure the existence of this shrinker. > > >> + /* >> + * We have already exited the read-side of rcu critical section >> + * before calling do_shrink_slab(), the shrinker_info may be >> + * released in expand_one_shrinker_info(), so reacquire the >> + * shrinker_info. >> + */ >> + index++; >> + goto again; > > With that, what makes the use of shrinker_info in > xchg_nr_deferred_memcg() in do_shrink_slab() coherent and valid? Holding rcu lock can ensure that the old shrinker_info will not be freed, and the shrinker_info_unit::nr_deferred can also be indexed from the old shrinker_info::unit[x], so the updated nr_deferred will not be lost. Thanks, Qi > > -Dave.
diff --git a/mm/shrinker.c b/mm/shrinker.c index d318f5621862..fee6f62904fb 100644 --- a/mm/shrinker.c +++ b/mm/shrinker.c @@ -107,6 +107,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg, lockdep_is_held(&shrinker_rwsem)); } +static struct shrinker_info *shrinker_info_rcu(struct mem_cgroup *memcg, + int nid) +{ + return rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); +} + static int expand_one_shrinker_info(struct mem_cgroup *memcg, int new_size, int old_size, int new_nr_max) { @@ -198,7 +204,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) struct shrinker_info_unit *unit; rcu_read_lock(); - info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info); + info = shrinker_info_rcu(memcg, nid); unit = info->unit[shriner_id_to_index(shrinker_id)]; if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) { /* Pairs with smp mb in shrink_slab() */ @@ -211,7 +217,7 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id) static DEFINE_IDR(shrinker_idr); -static int prealloc_memcg_shrinker(struct shrinker *shrinker) +static int shrinker_memcg_alloc(struct shrinker *shrinker) { int id, ret = -ENOMEM; @@ -219,7 +225,6 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker) return -ENOSYS; down_write(&shrinker_rwsem); - /* This may call shrinker, so it must use down_read_trylock() */ id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL); if (id < 0) goto unlock; @@ -237,7 +242,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker) return ret; } -static void unregister_memcg_shrinker(struct shrinker *shrinker) +static void shrinker_memcg_remove(struct shrinker *shrinker) { int id = shrinker->id; @@ -253,10 +258,15 @@ static long xchg_nr_deferred_memcg(int nid, struct shrinker *shrinker, { struct shrinker_info *info; struct shrinker_info_unit *unit; + long nr_deferred; - info = shrinker_info_protected(memcg, nid); + rcu_read_lock(); + info = shrinker_info_rcu(memcg, nid); unit = info->unit[shriner_id_to_index(shrinker->id)]; - return atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0); + nr_deferred = atomic_long_xchg(&unit->nr_deferred[shriner_id_to_offset(shrinker->id)], 0); + rcu_read_unlock(); + + return nr_deferred; } static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker, @@ -264,10 +274,16 @@ static long add_nr_deferred_memcg(long nr, int nid, struct shrinker *shrinker, { struct shrinker_info *info; struct shrinker_info_unit *unit; + long nr_deferred; - info = shrinker_info_protected(memcg, nid); + rcu_read_lock(); + info = shrinker_info_rcu(memcg, nid); unit = info->unit[shriner_id_to_index(shrinker->id)]; - return atomic_long_add_return(nr, &unit->nr_deferred[shriner_id_to_offset(shrinker->id)]); + nr_deferred = + atomic_long_add_return(nr, &unit->nr_deferred[shriner_id_to_offset(shrinker->id)]); + rcu_read_unlock(); + + return nr_deferred; } void reparent_shrinker_deferred(struct mem_cgroup *memcg) @@ -299,12 +315,12 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg) up_read(&shrinker_rwsem); } #else -static int prealloc_memcg_shrinker(struct shrinker *shrinker) +static int shrinker_memcg_alloc(struct shrinker *shrinker) { return -ENOSYS; } -static void unregister_memcg_shrinker(struct shrinker *shrinker) +static void shrinker_memcg_remove(struct shrinker *shrinker) { } @@ -464,18 +480,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, if (!mem_cgroup_online(memcg)) return 0; - if (!down_read_trylock(&shrinker_rwsem)) - return 0; - - info = shrinker_info_protected(memcg, nid); +again: + rcu_read_lock(); + info = shrinker_info_rcu(memcg, nid); if (unlikely(!info)) goto unlock; - for (; index < shriner_id_to_index(info->map_nr_max); index++) { + if (index < shriner_id_to_index(info->map_nr_max)) { struct shrinker_info_unit *unit; unit = info->unit[index]; + /* + * The shrinker_info_unit will not be freed, so we can + * safely release the RCU lock here. + */ + rcu_read_unlock(); + for_each_set_bit(offset, unit->map, SHRINKER_UNIT_BITS) { struct shrink_control sc = { .gfp_mask = gfp_mask, @@ -485,12 +506,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, struct shrinker *shrinker; int shrinker_id = calc_shrinker_id(index, offset); + rcu_read_lock(); shrinker = idr_find(&shrinker_idr, shrinker_id); - if (unlikely(!shrinker || !(shrinker->flags & SHRINKER_REGISTERED))) { - if (!shrinker) - clear_bit(offset, unit->map); + if (unlikely(!shrinker || !shrinker_try_get(shrinker))) { + clear_bit(offset, unit->map); + rcu_read_unlock(); continue; } + rcu_read_unlock(); /* Call non-slab shrinkers even though kmem is disabled */ if (!memcg_kmem_online() && @@ -523,15 +546,20 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, set_shrinker_bit(memcg, nid, shrinker_id); } freed += ret; - - if (rwsem_is_contended(&shrinker_rwsem)) { - freed = freed ? : 1; - goto unlock; - } + shrinker_put(shrinker); } + + /* + * We have already exited the read-side of rcu critical section + * before calling do_shrink_slab(), the shrinker_info may be + * released in expand_one_shrinker_info(), so reacquire the + * shrinker_info. + */ + index++; + goto again; } unlock: - up_read(&shrinker_rwsem); + rcu_read_unlock(); return freed; } #else /* !CONFIG_MEMCG */ @@ -638,7 +666,7 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...) shrinker->flags = flags | SHRINKER_ALLOCATED; if (flags & SHRINKER_MEMCG_AWARE) { - err = prealloc_memcg_shrinker(shrinker); + err = shrinker_memcg_alloc(shrinker); if (err == -ENOSYS) shrinker->flags &= ~SHRINKER_MEMCG_AWARE; else if (err == 0) @@ -731,7 +759,7 @@ void shrinker_free(struct shrinker *shrinker) } if (shrinker->flags & SHRINKER_MEMCG_AWARE) - unregister_memcg_shrinker(shrinker); + shrinker_memcg_remove(shrinker); up_write(&shrinker_rwsem); if (debugfs_entry)
Like global slab shrink, this commit also uses refcount+RCU method to make memcg slab shrink lockless. Use the following script to do slab shrink stress test: ``` DIR="/root/shrinker/memcg/mnt" do_create() { mkdir -p /sys/fs/cgroup/memory/test echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes for i in `seq 0 $1`; do mkdir -p /sys/fs/cgroup/memory/test/$i; echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; mkdir -p $DIR/$i; done } do_mount() { for i in `seq $1 $2`; do mount -t tmpfs $i $DIR/$i; done } do_touch() { for i in `seq $1 $2`; do echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs; dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 & done } case "$1" in touch) do_touch $2 $3 ;; test) do_create 4000 do_mount 0 4000 do_touch 0 3000 ;; *) exit 1 ;; esac ``` Save the above script, then run test and touch commands. Then we can use the following perf command to view hotspots: perf top -U -F 999 1) Before applying this patchset: 40.44% [kernel] [k] down_read_trylock 17.59% [kernel] [k] up_read 13.64% [kernel] [k] pv_native_safe_halt 11.90% [kernel] [k] shrink_slab 8.21% [kernel] [k] idr_find 2.71% [kernel] [k] _find_next_bit 1.36% [kernel] [k] shrink_node 0.81% [kernel] [k] shrink_lruvec 0.80% [kernel] [k] __radix_tree_lookup 0.50% [kernel] [k] do_shrink_slab 0.21% [kernel] [k] list_lru_count_one 0.16% [kernel] [k] mem_cgroup_iter 2) After applying this patchset: 60.17% [kernel] [k] shrink_slab 20.42% [kernel] [k] pv_native_safe_halt 3.03% [kernel] [k] do_shrink_slab 2.73% [kernel] [k] shrink_node 2.27% [kernel] [k] shrink_lruvec 2.00% [kernel] [k] __rcu_read_unlock 1.92% [kernel] [k] mem_cgroup_iter 0.98% [kernel] [k] __rcu_read_lock 0.91% [kernel] [k] osq_lock 0.63% [kernel] [k] mem_cgroup_calculate_protection 0.55% [kernel] [k] shrinker_put 0.46% [kernel] [k] list_lru_count_one We can see that the first perf hotspot becomes shrink_slab, which is what we expect. Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> --- mm/shrinker.c | 80 ++++++++++++++++++++++++++++++++++----------------- 1 file changed, 54 insertions(+), 26 deletions(-)