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

[Yosry Ahmed]

On Thu, Mar 31, 2022 at 8:38 PM Wei Xu <weixugc@xxxxxxxxxx> wrote:
>
> On Thu, Mar 31, 2022 at 5:33 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, 31 Mar 2022 08:41:51 +0000 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> >
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -6355,6 +6355,38 @@ static ssize_t memory_oom_group_write(struct kernfs_open_file *of,
> > >       return nbytes;
> > >  }
> > >
> > > +static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
> > > +                           size_t nbytes, loff_t off)
> > > +{
> > > +     struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
> > > +     unsigned int nr_retries = MAX_RECLAIM_RETRIES;
> > > +     unsigned long nr_to_reclaim, nr_reclaimed = 0;
> > > +     int err;
> > > +
> > > +     buf = strstrip(buf);
> > > +     err = page_counter_memparse(buf, "", &nr_to_reclaim);
> > > +     if (err)
> > > +             return err;
> > > +
> > > +     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;
> > > +     }
> >
> > Is there any way in which this can be provoked into triggering the
> > softlockup detector?
>
> memory.reclaim is similar to memory.high w.r.t. reclaiming memory,
> except that memory.reclaim is stateless, while the kernel remembers
> the state set by memory.high.  So memory.reclaim should not bring in
> any new risks of triggering soft lockup, if any.
>
> > Is it optimal to do the MAX_RECLAIM_RETRIES loop in the kernel?
> > Would additional flexibility be gained by letting userspace handle
> > retrying?
>
> I agree it is better to retry from the userspace.

Thanks Andrew and Wei for looking at this. IIUC the
MAX_RECLAIM_RETRIES loop was modeled after the loop in memory.high as
well. Is there a reason why it should be different here?