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Date:   Wed, 30 Aug 2017 11:55:24 +0100
From:   Roman Gushchin <guro@...com>
To:     Johannes Weiner <hannes@...xchg.org>
CC:     <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        <cgroups@...r.kernel.org>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: memcontrol: use per-cpu stocks for socket memory
 uncharging

On Tue, Aug 29, 2017 at 03:26:21PM -0400, Johannes Weiner wrote:
> On Tue, Aug 29, 2017 at 11:01:50AM +0100, Roman Gushchin wrote:
> > We've noticed a quite sensible performance overhead on some hosts
> > with significant network traffic when socket memory accounting
> > is enabled.
> > 
> > Perf top shows that socket memory uncharging path is hot:
> >   2.13%  [kernel]                [k] page_counter_cancel
> >   1.14%  [kernel]                [k] __sk_mem_reduce_allocated
> >   1.14%  [kernel]                [k] _raw_spin_lock
> >   0.87%  [kernel]                [k] _raw_spin_lock_irqsave
> >   0.84%  [kernel]                [k] tcp_ack
> >   0.84%  [kernel]                [k] ixgbe_poll
> >   0.83%  < workload >
> >   0.82%  [kernel]                [k] enqueue_entity
> >   0.68%  [kernel]                [k] __fget
> >   0.68%  [kernel]                [k] tcp_delack_timer_handler
> >   0.67%  [kernel]                [k] __schedule
> >   0.60%  < workload >
> >   0.59%  [kernel]                [k] __inet6_lookup_established
> >   0.55%  [kernel]                [k] __switch_to
> >   0.55%  [kernel]                [k] menu_select
> >   0.54%  libc-2.20.so            [.] __memcpy_avx_unaligned
> > 
> > To address this issue, the existing per-cpu stock infrastructure
> > can be used.
> > 
> > refill_stock() can be called from mem_cgroup_uncharge_skmem()
> > to move charge to a per-cpu stock instead of calling atomic
> > page_counter_uncharge().
> > 
> > To prevent the uncontrolled growth of per-cpu stocks,
> > refill_stock() will explicitly drain the cached charge,
> > if the cached value exceeds CHARGE_BATCH.
> > 
> > This allows significantly optimize the load:
> >   1.21%  [kernel]                [k] _raw_spin_lock
> >   1.01%  [kernel]                [k] ixgbe_poll
> >   0.92%  [kernel]                [k] _raw_spin_lock_irqsave
> >   0.90%  [kernel]                [k] enqueue_entity
> >   0.86%  [kernel]                [k] tcp_ack
> >   0.85%  < workload >
> >   0.74%  perf-11120.map          [.] 0x000000000061bf24
> >   0.73%  [kernel]                [k] __schedule
> >   0.67%  [kernel]                [k] __fget
> >   0.63%  [kernel]                [k] __inet6_lookup_established
> >   0.62%  [kernel]                [k] menu_select
> >   0.59%  < workload >
> >   0.59%  [kernel]                [k] __switch_to
> >   0.57%  libc-2.20.so            [.] __memcpy_avx_unaligned
> > 
> > Signed-off-by: Roman Gushchin <guro@...com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Michal Hocko <mhocko@...nel.org>
> > Cc: Vladimir Davydov <vdavydov.dev@...il.com>
> > Cc: cgroups@...r.kernel.org
> > Cc: kernel-team@...com
> > Cc: linux-mm@...ck.org
> > Cc: linux-kernel@...r.kernel.org
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> Neat!
> 
> As far as other types of pages go: page cache and anon are already
> batched pretty well, but I think kmem might benefit from this
> too. Have you considered using the stock in memcg_kmem_uncharge()?

Good idea!
I'll try to find an appropriate testcase and check if it really
brings any benefits. If so, I'll master a patch.

Thanks!

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