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SubjectRe: [PATCH 3/3] mm: Count list_lru_one::nr_items lockless
On Tue, Aug 22, 2017 at 03:29:35PM +0300, Kirill Tkhai wrote:
> During the reclaiming slab of a memcg, shrink_slab iterates
> over all registered shrinkers in the system, and tries to count
> and consume objects related to the cgroup. In case of memory
> pressure, this behaves bad: I observe high system time and
> time spent in list_lru_count_one() for many processes on RHEL7
> kernel (collected via $perf record --call-graph fp -j k -a):
>
> 0,50% nixstatsagent [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
> 0,26% nixstatsagent [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
> 0,23% nixstatsagent [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
> 0,15% nixstatsagent [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
> 0,15% nixstatsagent [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
>
> 0,94% mysqld [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
> 0,57% mysqld [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
> 0,51% mysqld [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
> 0,32% mysqld [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
> 0,32% mysqld [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2
>
> 0,73% sshd [kernel.vmlinux] [k] _raw_spin_lock [k] _raw_spin_lock
> 0,35% sshd [kernel.vmlinux] [k] shrink_slab [k] shrink_slab
> 0,32% sshd [kernel.vmlinux] [k] super_cache_count [k] super_cache_count
> 0,21% sshd [kernel.vmlinux] [k] __list_lru_count_one.isra.2 [k] _raw_spin_lock
> 0,21% sshd [kernel.vmlinux] [k] list_lru_count_one [k] __list_lru_count_one.isra.2

It would be nice to see how this is improved by this patch.
Can you try to record the traces on the vanilla kernel with
and without this patch?

>
> This patch aims to make super_cache_count() more effective. It
> makes __list_lru_count_one() count nr_items lockless to minimize
> overhead introducing by locking operation, and to make parallel
> reclaims more scalable.
>
> The lock won't be taken on shrinker::count_objects(),
> it would be taken only for the real shrink by the thread,
> who realizes it.
>

> https://jira.sw.ru/browse/PSBM-69296

Not relevant.

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