Message ID | 1676328eb3e9d02a4432783494d0c5a13f0d4656.1428622095.git.tgraf@suug.ch |
---|---|
State | Superseded, archived |
Delegated to: | David Miller |
Headers | show |
On Fri, 2015-04-10 at 01:43 +0200, Thomas Graf wrote: > Makes use of the __alignof__ GCC builtin to determine the minimal > amount of padding required to align the pointer afterwards. > > Due to use of ____cacheline_aligned_in_smp inside net_device, > struct net_device is actually always a multiple of SMP_CACHE_BYTES > so typically no padding is needed but this logic is kept in place > in case that is no longer the case. > > Signed-off-by: Thomas Graf <tgraf@suug.ch> > --- > include/linux/kernel.h | 2 ++ > net/core/dev.c | 3 +-- > 2 files changed, 3 insertions(+), 2 deletions(-) > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > index d6d630d..4ec018f 100644 > --- a/include/linux/kernel.h > +++ b/include/linux/kernel.h > @@ -50,6 +50,8 @@ > #define __ALIGN_MASK(x, mask) __ALIGN_KERNEL_MASK((x), (mask)) > #define PTR_ALIGN(p, a) ((typeof(p))ALIGN((unsigned long)(p), (a))) > #define IS_ALIGNED(x, a) (((x) & ((typeof(x))(a) - 1)) == 0) > +#define PTR_ALIGN_PAD(type, a) \ > + ({ (__alignof__(type) < (a)) ? (a) - __alignof__(type) : 0; }) > > #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr)) > > diff --git a/net/core/dev.c b/net/core/dev.c > index b2775f0..2b43aba 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -6794,8 +6794,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name, > alloc_size = ALIGN(alloc_size, NETDEV_ALIGN); > alloc_size += sizeof_priv; > } > - /* ensure 32-byte alignment of whole construct */ > - alloc_size += NETDEV_ALIGN - 1; > + alloc_size += PTR_ALIGN_PAD(struct net_device, NETDEV_ALIGN); > > p = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT); > if (!p) I believe code intent was to get an alignment even if kmalloc() returns say 0xxxxx0008. AFAIK, SLAB in debug mode was able to do this. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 04/09/15 at 05:34pm, Eric Dumazet wrote: > On Fri, 2015-04-10 at 01:43 +0200, Thomas Graf wrote: > > diff --git a/net/core/dev.c b/net/core/dev.c > > index b2775f0..2b43aba 100644 > > --- a/net/core/dev.c > > +++ b/net/core/dev.c > > @@ -6794,8 +6794,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name, > > alloc_size = ALIGN(alloc_size, NETDEV_ALIGN); > > alloc_size += sizeof_priv; > > } > > - /* ensure 32-byte alignment of whole construct */ > > - alloc_size += NETDEV_ALIGN - 1; > > + alloc_size += PTR_ALIGN_PAD(struct net_device, NETDEV_ALIGN); > > > > p = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT); > > if (!p) > > > I believe code intent was to get an alignment even if kmalloc() returns > say 0xxxxx0008. > > AFAIK, SLAB in debug mode was able to do this. I assumed kmalloc would guarantee alignment of the return pointer as indicated by __alignof__ but I now realize that this can't possibly be the case. kmalloc only knows about the size. We'll have to drop patch 3. Fortunately it's still below 2K. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, 2015-04-10 at 09:48 +0100, Thomas Graf wrote: > > I assumed kmalloc would guarantee alignment of the return pointer as > indicated by __alignof__ but I now realize that this can't possibly be the > case. kmalloc only knows about the size. We'll have to drop patch 3. > Fortunately it's still below 2K. If you really care for this, take a look at qdisc_alloc() and commit d276055c4e90a7278cd5167ba9755c9b214bcff7 ("net_sched: reduce fifo qdisc size") (Note that some setups require ~3 network devices but 3000+ qdisc) I guess it might be nice to define a common helper and reuse it. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/include/linux/kernel.h b/include/linux/kernel.h index d6d630d..4ec018f 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -50,6 +50,8 @@ #define __ALIGN_MASK(x, mask) __ALIGN_KERNEL_MASK((x), (mask)) #define PTR_ALIGN(p, a) ((typeof(p))ALIGN((unsigned long)(p), (a))) #define IS_ALIGNED(x, a) (((x) & ((typeof(x))(a) - 1)) == 0) +#define PTR_ALIGN_PAD(type, a) \ + ({ (__alignof__(type) < (a)) ? (a) - __alignof__(type) : 0; }) #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr)) diff --git a/net/core/dev.c b/net/core/dev.c index b2775f0..2b43aba 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -6794,8 +6794,7 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name, alloc_size = ALIGN(alloc_size, NETDEV_ALIGN); alloc_size += sizeof_priv; } - /* ensure 32-byte alignment of whole construct */ - alloc_size += NETDEV_ALIGN - 1; + alloc_size += PTR_ALIGN_PAD(struct net_device, NETDEV_ALIGN); p = kzalloc(alloc_size, GFP_KERNEL | __GFP_NOWARN | __GFP_REPEAT); if (!p)
Makes use of the __alignof__ GCC builtin to determine the minimal amount of padding required to align the pointer afterwards. Due to use of ____cacheline_aligned_in_smp inside net_device, struct net_device is actually always a multiple of SMP_CACHE_BYTES so typically no padding is needed but this logic is kept in place in case that is no longer the case. Signed-off-by: Thomas Graf <tgraf@suug.ch> --- include/linux/kernel.h | 2 ++ net/core/dev.c | 3 +-- 2 files changed, 3 insertions(+), 2 deletions(-)