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

[Michal Hocko <mhocko@suse.cz>]

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.

> Thanks,
> -Kame

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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