linux-kernel - Re: [PATCH 4/6] mm/vmscan: remove redundant current_may

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Date:   Tue, 20 Mar 2018 16:11:15 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH 4/6] mm/vmscan: remove redundant current_may_throttle()
 check

On Thu 15-03-18 19:45:51, Andrey Ryabinin wrote:
> Only kswapd can have non-zero nr_immediate, and current_may_throttle() is
> always true for kswapd (PF_LESS_THROTTLE bit is never set) thus it's
> enough to check stat.nr_immediate only.

OK, so this is a result of some code evolution. We used to check for
dirty pages as well. But then b738d764652d ("Don't trigger congestion
wait on dirty-but-not-writeout pages") removed the nr_unqueued_dirty ==
nr_taken part. I was wondering whether we still have the
PF_LESS_THROTTLE protection in place but then noticed that we still have
	if (!sc->hibernation_mode && !current_is_kswapd() &&
	    current_may_throttle())
		wait_iff_congested(pgdat, BLK_RW_ASYNC, HZ/10);

in place, so good.

> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/vmscan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0d5ab312a7f4..a8f6e4882e00 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1806,7 +1806,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
>  		 * that pages are cycling through the LRU faster than
>  		 * they are written so also forcibly stall.
>  		 */
> -		if (stat.nr_immediate && current_may_throttle())
> +		if (stat.nr_immediate)
>  			congestion_wait(BLK_RW_ASYNC, HZ/10);
>  	}
>  
> -- 
> 2.16.1

-- 
Michal Hocko
SUSE Labs