Re: [PATCH] mm: memcg: yield cpu when we fail to charge pages

From: "Julius Hemanth Pitti (jpitti)" <jpitti@cisco.com>
To: "guro@fb.com" <guro@fb.com>
Cc: "vdavydov.dev@gmail.com" <vdavydov.dev@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"hannes@cmpxchg.org" <hannes@cmpxchg.org>,
	"xe-linux-external(mailer list)" <xe-linux-external@cisco.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"mhocko@kernel.org" <mhocko@kernel.org>
Subject: Re: [PATCH] mm: memcg: yield cpu when we fail to charge pages
Date: Tue, 8 Sep 2020 19:28:51 +0000	[thread overview]
Message-ID: <38c678cac5794cb647ba73799bcaf6df5ff6dc3e.camel@cisco.com> (raw)
In-Reply-To: <20200908192159.GB567128@carbon.DHCP.thefacebook.com>

On Tue, 2020-09-08 at 12:21 -0700, Roman Gushchin wrote:
> On Tue, Sep 08, 2020 at 11:50:51AM -0700, Julius Hemanth Pitti wrote:
> > For non root CG, in try_charge(), we keep trying
> > to charge until we succeed. On non-preemptive
> > kernel, when we are OOM, this results in holding
> > CPU forever.
> > 
> > On SMP systems, this doesn't create a big problem
> > because oom_reaper get a change to kill victim
> > and make some free pages. However on a single-core
> > CPU (or cases where oom_reaper pinned to same CPU
> > where try_charge is executing), oom_reaper shall
> > never get scheduled and we stay in try_charge forever.
> > 
> > Steps to repo this on non-smp:
> > 1. mount -t tmpfs none /sys/fs/cgroup
> > 2. mkdir /sys/fs/cgroup/memory
> > 3. mount -t cgroup none /sys/fs/cgroup/memory -o memory
> > 4. mkdir /sys/fs/cgroup/memory/0
> > 5. echo 40M > /sys/fs/cgroup/memory/0/memory.limit_in_bytes
> > 6. echo $$ > /sys/fs/cgroup/memory/0/tasks
> > 7. stress -m 5 --vm-bytes 10M --vm-hang 0
> > 
> > Signed-off-by: Julius Hemanth Pitti <jpitti@cisco.com>
> > ---
> >  mm/memcontrol.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 0d6f3ea86738..4620d70267cb 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2652,6 +2652,8 @@ static int try_charge(struct mem_cgroup
> > *memcg, gfp_t gfp_mask,
> >  	if (fatal_signal_pending(current))
> >  		goto force;
> >  
> > +	cond_resched();
> > +
> 
> Can you, please, add a short comment here?
> Something like "give oom_reaper a chance on a non-SMP system"?
Sure.

> 
> >  	/*
> >  	 * keep retrying as long as the memcg oom killer is able to
> > make
> >  	 * a forward progress or bypass the charge if the oom killer
> > -- 
> > 2.17.1
> > 
> 
> The patch makes total sense to me. Please, feel free to add
> Acked-by: Roman Gushchin <guro@fb.com> after adding a comment.
Thanks, I shall add.

> 
> Thank you!