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Date:   Tue, 8 Sep 2020 12:21:59 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Julius Hemanth Pitti <jpitti@cisco.com>
CC:     <hannes@cmpxchg.org>, <mhocko@kernel.org>,
        <vdavydov.dev@gmail.com>, <akpm@linux-foundation.org>,
        <cgroups@vger.kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <xe-linux-external@cisco.com>
Subject: Re: [PATCH] mm: memcg: yield cpu when we fail to charge pages
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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"?

>  	/*
>  	 * 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.

Thank you!