linux-kernel - Re: [PATCH] mm: memcontrol: use per-cpu stocks for socket memory uncharging

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Date:   Tue, 29 Aug 2017 15:26:21 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
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 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()?