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From: Balbir Singh <bsingharora@gmail.com>
Date: Mon, 5 Dec 2016 23:44:27 +1100
Message-ID: <CAKTCnz=K8QG69tKB8yStiZypBzcvnE=wW+25xuo9f_HZNzPtDg@mail.gmail.com>
Subject: Re: [PATCH] mm, vmscan: add cond_resched into shrink_node_memcg
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>,
        Boris Zhmurov <bb@kernelpanic.ru>,
        "Christopher S. Aker" <caker@theshore.net>,
        Donald Buczek <buczek@molgen.mpg.de>,
        Paul Menzel <pmenzel@molgen.mpg.de>
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>
> Hi,
> there were multiple reportes of the similar RCU stalls. Only Boris has
> confirmed that this patch helps in his workload. Others might see a
> slightly different issue and that should be investigated if it is the
> case. As pointed out by Paul [1] cond_resched might be not sufficient
> to silence RCU stalls because that would require a real scheduling.
> This is a separate problem, though, and Paul is working with Peter [2]
> to resolve it.
>
> Anyway, I believe that this patch should be a good start because it
> really seems that nr_taken=0 during the LRU isolation can be triggered
> in the real life. All reporters are agreeing to start seeing this issue
> when moving on to 4.8 kernel which might be just a coincidence or a
> different behavior of some subsystem. Well, MM has moved from zone to
> node reclaim but I couldn't have found any direct relation to that
> change.
>
> [1] http://lkml.kernel.org/r/20161130142955.GS3924@linux.vnet.ibm.com
> [2] http://lkml.kernel.org/r/20161201124024.GB3924@linux.vnet.ibm.com
>
>  mm/vmscan.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c05f00042430..c4abf08861d2 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2362,6 +2362,8 @@ static void shrink_node_memcg(struct pglist_data *pgdat, struct mem_cgroup *memc
>                         }
>                 }
>
> +               cond_resched();
> +

I see a cond_resched_rcu_qs() as a part of linux next inside the while
(nr[..]) loop.
Do we need this as well?

Balbir Singh.