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

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Apologies for the delayed response,

Yosry Ahmed <yosryahmed@google.com> writes:

> On Fri, Apr 1, 2022 at 1:39 AM Vaibhav Jain <vaibhav@linux.ibm.com> wrote:
>>
>>
>> Yosry Ahmed <yosryahmed@google.com> writes:
>> > From: Shakeel Butt <shakeelb@google.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