Re: [PATCH 03/10] mm: Assign memcg-aware shrinkers bitmap to memcg

[Matthew Wilcox]

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.