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    SubjectRe: [PATCH] mm: memcontrol: use per-cpu stocks for socket memory uncharging
    On Wed 30-08-17 13:57:29, Roman Gushchin wrote:
    > On Wed, Aug 30, 2017 at 02:55:43PM +0200, Michal Hocko wrote:
    > > On Wed 30-08-17 13:44:59, Roman Gushchin wrote:
    > > > On Wed, Aug 30, 2017 at 02:36:55PM +0200, Michal Hocko wrote:
    > > > > On Tue 29-08-17 11:01:50, Roman Gushchin wrote:
    > > > > [...]
    > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > > > > > index b9cf3cf4a3d0..a69d23082abf 100644
    > > > > > --- a/mm/memcontrol.c
    > > > > > +++ b/mm/memcontrol.c
    > > > > > @@ -1792,6 +1792,9 @@ static void refill_stock(struct mem_cgroup *memcg, unsigned int nr_pages)
    > > > > > }
    > > > > > stock->nr_pages += nr_pages;
    > > > > >
    > > > > > + if (stock->nr_pages > CHARGE_BATCH)
    > > > > > + drain_stock(stock);
    > > > > > +
    > > > > > local_irq_restore(flags);
    > > > > > }
    > > > >
    > > > > Why do we need this? In other words, why cannot we rely on draining we
    > > > > already do?
    > > >
    > > > The existing draining depends on memory pressure, so to keep
    > > > the accounting (which we expose to a user) reasonable accurate
    > > > even without memory pressure, we need to limit the size of per-cpu stocks.
    > >
    > > Why don't we need this for regular page charges? Or maybe we do but that
    > > sounds like a seprate and an unrealted fix to me.
    >
    > Because we never refill more than CHARGE_BATCH.

    You are right that a single process will not but try_charge is a preemptible
    context and so multiple processes might pass consume_stock and then
    charge a N*CHARGE_BATCH. But I agree that this is quite unlikely so a
    separate patch is probably not worth it.

    --
    Michal Hocko
    SUSE Labs

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