linux-kernel - Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

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Date:   Mon, 04 Apr 2022 09:20:16 +0530
From:   Vaibhav Jain <vaibhav@...ux.ibm.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        Jonathan Corbet <corbet@....net>, Yu Zhao <yuzhao@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH resend] memcg: introduce per-memcg reclaim interface


Apologies for the delayed response,

Yosry Ahmed <yosryahmed@...gle.com> writes:

> On Fri, Apr 1, 2022 at 1:39 AM Vaibhav Jain <vaibhav@...ux.ibm.com> wrote:
>>
>>
>> Yosry Ahmed <yosryahmed@...gle.com> writes:
>> > From: Shakeel Butt <shakeelb@...gle.com>
>> >
>> > Introduce an memcg interface to trigger memory reclaim on a memory cgroup.
>> <snip>
>>
>> > +
>> > +     while (nr_reclaimed < nr_to_reclaim) {
>> > +             unsigned long reclaimed;
>> > +
>> > +             if (signal_pending(current))
>> > +                     break;
>> > +
>> > +             reclaimed = try_to_free_mem_cgroup_pages(memcg,
>> > +                                             nr_to_reclaim - nr_reclaimed,
>> > +                                             GFP_KERNEL, true);
>> > +
>> > +             if (!reclaimed && !nr_retries--)
>> > +                     break;
>> > +
>> > +             nr_reclaimed += reclaimed;
>>
>> I think there should be a cond_resched() in this loop before
>> try_to_free_mem_cgroup_pages() to have better chances of reclaim
>> succeding early.
>>
> Thanks for taking the time to look at this!
>
> I believe this loop is modeled after the loop in memory_high_write()
> for the memory.high interface. Is there a reason why it should be
> needed here but not there?
>

memory_high_write() calls drain_all_stock() atleast once before calling
try_to_free_mem_cgroup_pages(). This would drain all percpu stocks
for the given memcg and its descendents, giving a high chance
try_to_free_mem_cgroup_pages() to succeed quickly. Such a functionality
is missing from this patch.

Adding a cond_resched() would atleast give chance to other processess
within the memcg to run and make forward progress thereby making more
pages available for reclaim.

Suggestion is partly based on __perform_reclaim() issues a cond_resche()
as it may get called repeatedly during direct reclaim path.


>> <snip>
>>
>> --
>> Cheers
>> ~ Vaibhav
>

-- 
Cheers
~ Vaibhav