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Subject: Re: [PATCH 03/10] mm: Assign memcg-aware shrinkers bitmap to memcg
To:     Matthew Wilcox <willy@infradead.org>
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From:   Kirill Tkhai <ktkhai@virtuozzo.com>
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Date:   Wed, 21 Mar 2018 18:43:01 +0300
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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.

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.

> 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.

Thanks,
Kirill