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

[Yosry Ahmed <yosryahmed@google.com>]

On Sun, Apr 3, 2022 at 8:50 PM Vaibhav Jain <vaibhav@linux.ibm.com> wrote:
>
>
> 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.
>
As Michal pointed out, there is already a call to cond_resched() in
shrink_node_memcgs().
>
> >> <snip>
> >>
> >> --
> >> Cheers
> >> ~ Vaibhav
> >
>
> --
> Cheers
> ~ Vaibhav