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

[Kirill Tkhai]

On 22.08.2017 22:47, Vladimir Davydov wrote:
> 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?

Sadly, the talk is about a production node, and it's impossible to use vanila kernel there.

>>
>> 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.
>