linux-kernel - Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback completes if congested

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Date:   Tue, 21 Sep 2021 10:13:17 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Mel Gorman" <mgorman@...hsingularity.net>
Cc:     "Linux-MM" <linux-mm@...ck.org>, "Theodore Ts'o" <tytso@....edu>,
        "Andreas Dilger" <adilger.kernel@...ger.ca>,
        "Darrick J . Wong" <djwong@...nel.org>,
        "Matthew Wilcox" <willy@...radead.org>,
        "Michal Hocko" <mhocko@...e.com>,
        "Dave Chinner" <david@...morbit.com>,
        "Rik van Riel" <riel@...riel.com>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Johannes Weiner" <hannes@...xchg.org>,
        "Jonathan Corbet" <corbet@....net>,
        "Linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Mel Gorman" <mgorman@...hsingularity.net>
Subject: Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback
 completes if congested

On Mon, 20 Sep 2021, Mel Gorman wrote:
> -long wait_iff_congested(int sync, long timeout)
> -{
> -	long ret;
> -	unsigned long start = jiffies;
> -	DEFINE_WAIT(wait);
> -	wait_queue_head_t *wqh = &congestion_wqh[sync];
> -
> -	/*
> -	 * If there is no congestion, yield if necessary instead
> -	 * of sleeping on the congestion queue
> -	 */
> -	if (atomic_read(&nr_wb_congested[sync]) == 0) {
> -		cond_resched();
> -
> -		/* In case we scheduled, work out time remaining */
> -		ret = timeout - (jiffies - start);
> -		if (ret < 0)
> -			ret = 0;
> -
> -		goto out;
> -	}
> -
> -	/* Sleep until uncongested or a write happens */
> -	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);

Uninterruptible wait.

....
> +static void
> +reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
> +							long timeout)
> +{
> +	wait_queue_head_t *wqh = &pgdat->reclaim_wait;
> +	unsigned long start = jiffies;
> +	long ret;
> +	DEFINE_WAIT(wait);
> +
> +	atomic_inc(&pgdat->nr_reclaim_throttled);
> +	WRITE_ONCE(pgdat->nr_reclaim_start,
> +		 node_page_state(pgdat, NR_THROTTLED_WRITTEN));
> +
> +	prepare_to_wait(wqh, &wait, TASK_INTERRUPTIBLE);

Interruptible wait.

Why the change?  I think these waits really need to be TASK_UNINTERRUPTIBLE.

Thanks,
NeilBrown