linux-kernel - Re: [PATCH v2 2/2] mm, slab: Show last shrink time in us when slab/shrink is read

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Date:   Thu, 18 Jul 2019 10:36:59 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v2 2/2] mm, slab: Show last shrink time in us when
 slab/shrink is read

On 7/18/19 7:39 AM, Christopher Lameter wrote:
> On Wed, 17 Jul 2019, Waiman Long wrote:
>
>> The show method of /sys/kernel/slab/<slab>/shrink sysfs file currently
>> returns nothing. This is now modified to show the time of the last
>> cache shrink operation in us.
> What is this useful for? Any use cases?

I got query about how much time will the slab_mutex be held when
shrinking the cache. I don't have a solid answer as it depends on how
many memcg caches are there. This patch is a partial answer to that as
it give a rough upper bound of the lock hold time.

>> CONFIG_SLUB_DEBUG depends on CONFIG_SYSFS. So the new shrink_us field
>> is always available to the shrink methods.
> Aside from minimal systems without CONFIG_SYSFS... Does this build without
> CONFIG_SYSFS?

The sysfs code in mm/slub.c is guarded by CONFIG_SLUB_DEBUG which, in
turn, depends on CONFIG_SYSFS. So if CONFIG_SYSFS is off, the shrink
sysfs methods will be off as well. I haven't tried doing a minimal
build. I will certainly try that, but I don't expect any problem here.

Cheers,
Longman