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Subject: Re: [PATCH v2 2/2] mm, slab: Show last shrink time in us when
 slab/shrink is read
To:     Christopher Lameter <cl@linux.com>
Cc:     Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <guro@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>
References: <20190717202413.13237-1-longman@redhat.com>
 <20190717202413.13237-3-longman@redhat.com>
 <0100016c04e1562a-e516c595-1d46-40df-ab29-da1709277e9a-000000@email.amazonses.com>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <6fb9f679-02d1-c33f-2d79-4c2eaa45d264@redhat.com>
Date:   Thu, 18 Jul 2019 10:36:59 -0400
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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