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

Johannes Weiner
2017-08-29T19:26:21Z

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@fb.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: cgroups@vger.kernel.org
> Cc: kernel-team@fb.com
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org

Acked-by: Johannes Weiner <hannes@cmpxchg.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()?

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