For the last time, do not top post on netdev. On Tue, 2017-04-18 at 17:15 -0700, Code Soldier1 wrote: > 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 ? >
skb->cb is already 48 bytes, not 40. > If not then may I increase skb_shared_info -- However that would have > to be by 64bytes. You will have a very hard time to convince us that this 8 byte field is needed on all skbs, regardless of current sk_buff size. > > > 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 > >> > > >> > > >> > > >> > > >> > > >> > >> > >> > > > > > > >
