On Wed, Mar 21, 2018 at 06:43:01PM +0300, Kirill Tkhai wrote: > On 21.03.2018 18:26, Matthew Wilcox wrote: > > On Wed, Mar 21, 2018 at 06:12:17PM +0300, Kirill Tkhai wrote: > >> On 21.03.2018 17:56, Matthew Wilcox wrote: > >>> Why use your own bitmap here? Why not use an IDA which can grow and > >>> shrink automatically without you needing to play fun games with RCU? > >> > >> Bitmap allows to use unlocked set_bit()/clear_bit() to maintain the map > >> of not empty shrinkers. > >> > >> So, the reason to use IDR here is to save bitmap memory? Does this mean > >> IDA works fast with sparse identifiers? It seems they require per-memcg > >> lock to call IDR primitives. I just don't have information about this. > >> > >> If so, which IDA primitive can be used to set particular id in bitmap? > >> There is idr_alloc_cyclic(idr, NULL, id, id+1, GFP_KERNEL) only I see > >> to do that. > > > > You're confusing IDR and IDA in your email, which is unfortunate. > > > > You can set a bit in an IDA by calling ida_simple_get(ida, n, n, GFP_FOO); > > You clear it by calling ida_simple_remove(ida, n); > > I moved to IDR in the message, since IDA uses global spinlock. It will be > taken every time a first object is added to list_lru, or last is removed. > These may be frequently called operations, and they may scale not good > on big machines. I'm fixing the global spinlock issue with the IDA. Not going to be ready for 4.17, but hopefully for 4.18. > Using IDR will allow us to introduce memcg-related locks, but I'm still not > sure it's easy to introduce them in scalable-way. Simple set_bit()/clear_bit() > do not require locks at all. They're locked operations ... they may not have an explicit spinlock associated with them, but the locking still happens. > > The identifiers aren't going to be all that sparse; after all you're > > allocating them from a global IDA. Up to 62 identifiers will allocate > > no memory; 63-1024 identifiers will allocate a single 128 byte chunk. > > Between 1025 and 65536 identifiers, you'll allocate a 576-byte chunk > > and then 128-byte chunks for each block of 1024 identifiers (*). One of > > the big wins with the IDA is that it will shrink again after being used. > > I didn't read all the way through your patchset to see if you bother to > > shrink your bitmap after it's no longer used, but most resizing bitmaps > > we have in the kernel don't bother with that part. > > > > (*) Actually it's more complex than that... between 1025 and 1086, > > you'll have a 576 byte chunk, a 128-byte chunk and then use 62 bits of > > the next pointer before allocating a 128 byte chunk when reaching ID > > 1087. Similar things happen for the 62 bits after 2048, 3076 and so on. > > The individual chunks aren't shrunk until they're empty so if you set ID > > 1025 and then ID 1100, then clear ID 1100, the 128-byte chunk will remain > > allocated until ID 1025 is cleared. This probably doesn't matter to you. > > Sound great, thanks for explaining this. The big problem I see is > that IDA/IDR add primitives allocate memory, while they will be used > in the places, where they mustn't fail. There is list_lru_add(), and > it's called unconditionally in current kernel code. The patchset makes > the bitmap be populated in this function. So, we can't use IDR there. Maybe we can use GFP_NOFAIL here. They're small allocations, so we're only asking for single-page allocations to not fail, which shouldn't put too much strain on the VM.
