Re: [RFC] [PATCH 3/7 v2] memcg: remove PCG_MOVE_LOCK flag from pc->flags

[Ying Han <yinghan@google.com>]

On Wed, Jan 18, 2012 at 3:53 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> On Wed, 18 Jan 2012 11:47:03 +0100
> Michal Hocko <mhocko@suse.cz> wrote:
>
>> On Wed 18-01-12 09:12:26, KAMEZAWA Hiroyuki wrote:
>> > On Tue, 17 Jan 2012 17:46:05 +0100
>> > Michal Hocko <mhocko@suse.cz> wrote:
>> >
>> > > On Fri 13-01-12 17:40:19, KAMEZAWA Hiroyuki wrote:
>> [...]
>> > > > This patch removes PCG_MOVE_LOCK and add hashed rwlock array
>> > > > instead of it. This works well enough. Even when we need to
>> > > > take the lock,
>> > >
>> > > Hmmm, rwlocks are not popular these days very much.
>> > > Anyway, can we rather make it (source) memcg (bit)spinlock instead. We
>> > > would reduce false sharing this way and would penalize only pages from
>> > > the moving group.
>> > >
>> > per-memcg spinlock ?
>>
>> Yes
>>
>> > The reason I used rwlock() is to avoid disabling IRQ.  This routine
>> > will be called by IRQ context (for dirty ratio support).  So, IRQ
>> > disable will be required if we use spinlock.
>>
>> OK, I have missed the comment about disabling IRQs. It's true that we do
>> not have to be afraid about deadlocks if the lock is held only for
>> reading from the irq context but does the spinlock makes a performance
>> bottleneck? We are talking about the slowpath.
>> I could see the reason for the read lock when doing hashed locks because
>> they are global but if we make the lock per memcg then we shouldn't
>> interfere with other updates which are not blocked by the move.
>>
>
> Hm, ok. In the next version, I'll use per-memcg spinlock (with hash if necessary)

Just want to make sure I understand it, even we make the lock
per-memcg, there is still a false sharing of pc within one memcg. Do
we need to demonstrate the effect ?

Also, I don't get the point of why spinlock instead of rwlock in this case?

--Ying

>
> Thanks,
> -Kame
>
>

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