Re: [PATCH 3/3] mm: Count list_lru_one::nr_items lockless

[Vladimir Davydov]

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.