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

[Michal Hocko <mhocko@suse.cz>]

On Mon 23-01-12 14:05:33, Ying Han wrote:
> 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. 

Yes that is true. I have missed that we might fault in several pages at
once but this would happen only during task move, right? And that is not
a hot path anyway. Or?

> Do we need to demonstrate the effect ?
> 
> Also, I don't get the point of why spinlock instead of rwlock in this case?

spinlock provides a fairness while with rwlocks might lead to
starvation.


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

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