lkml.org 
[lkml]   [2011]   [Jul]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 3/4] memcg: get rid of percpu_charge_mutex lock
On Thu 21-07-11 19:30:51, KAMEZAWA Hiroyuki wrote:
> On Thu, 21 Jul 2011 09:58:24 +0200
> Michal Hocko <mhocko@suse.cz> wrote:
>
> > percpu_charge_mutex protects from multiple simultaneous per-cpu charge
> > caches draining because we might end up having too many work items.
> > At least this was the case until 26fe6168 (memcg: fix percpu cached
> > charge draining frequency) when we introduced a more targeted draining
> > for async mode.
> > Now that also sync draining is targeted we can safely remove mutex
> > because we will not send more work than the current number of CPUs.
> > FLUSHING_CACHED_CHARGE protects from sending the same work multiple
> > times and stock->nr_pages == 0 protects from pointless sending a work
> > if there is obviously nothing to be done. This is of course racy but we
> > can live with it as the race window is really small (we would have to
> > see FLUSHING_CACHED_CHARGE cleared while nr_pages would be still
> > non-zero).
> > The only remaining place where we can race is synchronous mode when we
> > rely on FLUSHING_CACHED_CHARGE test which might have been set by other
> > drainer on the same group but we should wait in that case as well.
> >
> > Signed-off-by: Michal Hocko <mhocko@suse.cz>
>
> A concern.
>
> > ---
> > mm/memcontrol.c | 12 ++----------
> > 1 files changed, 2 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 8180cd9..9d49a12 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2065,7 +2065,6 @@ struct memcg_stock_pcp {
> > #define FLUSHING_CACHED_CHARGE (0)
> > };
> > static DEFINE_PER_CPU(struct memcg_stock_pcp, memcg_stock);
> > -static DEFINE_MUTEX(percpu_charge_mutex);
> >
> > /*
> > * Try to consume stocked charge on this cpu. If success, one page is consumed
> > @@ -2166,7 +2165,8 @@ static void drain_all_stock(struct mem_cgroup *root_mem, bool sync)
> >
> > for_each_online_cpu(cpu) {
> > struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu);
> > - if (test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> > + if (root_mem == stock->cached &&
> > + test_bit(FLUSHING_CACHED_CHARGE, &stock->flags))
> > flush_work(&stock->work);
>
> Doesn't this new check handle hierarchy ?
> css_is_ancestor() will be required if you do this check.

Yes you are right. Will fix it. I will add a helper for the check.

> BTW, this change should be in other patch, I think.

I have put the change here intentionally because previously we were
protected by the lock so we couldn't race with somebody else so the
check was not necessary.

>
> Thanks,
> -Kame

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


\
 
 \ /
  Last update: 2011-07-21 13:49    [from the cache]
©2003-2011 Jasper Spaans