linux-kernel - Re: [PATCH 6/8] mm/vmscan: Centralise timeout values for reclaim

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Date:   Fri, 22 Oct 2021 12:06:13 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Mel Gorman" <mgorman@...hsingularity.net>
Cc:     "Andrew Morton" <akpm@...ux-foundation.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-MM" <linux-mm@...ck.org>,
        "Linux-fsdevel" <linux-fsdevel@...r.kernel.org>,
        "LKML" <linux-kernel@...r.kernel.org>,
        "Mel Gorman" <mgorman@...hsingularity.net>
Subject: Re: [PATCH 6/8] mm/vmscan: Centralise timeout values for reclaim_throttle

On Tue, 19 Oct 2021, Mel Gorman wrote:
...
> +	switch(reason) {
> +	case VMSCAN_THROTTLE_NOPROGRESS:
> +	case VMSCAN_THROTTLE_WRITEBACK:
> +		timeout = HZ/10;
> +
> +		if (atomic_inc_return(&pgdat->nr_writeback_throttled) == 1) {
> +			WRITE_ONCE(pgdat->nr_reclaim_start,
> +				node_page_state(pgdat, NR_THROTTLED_WRITTEN));

You have introduced a behaviour change that wasn't flagged in the commit
message.
Previously nr_writeback_throttled was only incremented for
VMSCAN_THROTTLE_WRITEBACK, now it is incremented for
VMSCAN_THROTTLE_NOPROGRESS as well.  

Some justification would be good.

> +		}
> +
> +		break;
> +	case VMSCAN_THROTTLE_ISOLATED:
> +		timeout = HZ/50;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
> +		timeout = HZ;
> +		break;
>  	}
>  
>  	prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
>  	ret = schedule_timeout(timeout);
>  	finish_wait(wqh, &wait);
>  
> -	if (acct_writeback)
> +	if (reason == VMSCAN_THROTTLE_ISOLATED)

(defect) I think you want "!=" there.

While the numbers a still magic, they are now well documented and all in
one place - a definite improvement!

Thanks,
NeilBrown