I am not suggesting that we do not do cache alignment. That is required for performance. I have gone through this exercise because I need to add a field to sk_buff and I want to do that without causing any adverse effects.
Now that we have discovered that there are 40 bytes that can be used without any adverse effect, may I increase skb->cb by 8 bytes ? If not then may I increase skb_shared_info -- However that would have to be by 64bytes. On Tue, Apr 18, 2017 at 4:55 PM, Eric Dumazet <eric.duma...@gmail.com> wrote: > Please do not top post on netdev > > On Tue, 2017-04-18 at 16:26 -0700, Code Soldier1 wrote: >> Eric, >> >> This alignment flag is passed to the cache constructor and the >> allocation is indeed cache aligned. However, since the allocated size >> is not a multiple of the alignment, wont memory be wasted ?. We can >> get 40 extra bytes without any side effects since they are on the same >> cache line ? > > We _want_ to align skb to cache lines. > > Fact that few bytes might be wasted is unfortunate, but negligible. > > BTW, if you do kmalloc(1025), kmalloc() wastes 1023 bytes. > > >> >> kmem_cache_create() code does an ALIGN() to round up the size. >> >> kasan_cache_create(cachep, &size, &flags); >> >> size = ALIGN(size, cachep->align); >> >> This is the size used in calculate_slab_order() to calculate num. I am >> assuming that in the non debug case gfp_order will be 0. >> >> Perhaps I am missing something. >> >> >> On Tue, Apr 18, 2017 at 12:00 PM, Eric Dumazet <eric.duma...@gmail.com> >> wrote: >> > On Tue, 2017-04-18 at 10:34 -0700, Code Soldier1 wrote: >> >> Hi Folks, >> >> >> >> I am sure there is a reason for the current sizes of these structures, >> >> However the reason is not obvious to me. So please help me understand. >> >> >> >> Currently the size of sk_buff on an x86_64 system is 232 bytes -- Why >> >> is that. I expected it to be a multiple of 32/64 as they are the most >> >> common cache lines. Since the alignment calculation will align the >> >> structure with the hw cache line, it seems like we might be wasting >> >> space ? >> >> >> >> skb_shared_info on the other hand is perfectly aligned with a size of 320 >> >> bytes. >> >> >> >> Thanks, >> >> >> > >> > The alignment is there. >> > Look at skb_init() code, using SLAB_HWCACHE_ALIGN >> > >> > >> > >> > >> > >> >> >> > > -- CS1
