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From:   "NeilBrown" <neilb@suse.de>
To:     "Mel Gorman" <mgorman@techsingularity.net>
Cc:     "Andrew Morton" <akpm@linux-foundation.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        "Andreas Dilger" <adilger.kernel@dilger.ca>,
        "Darrick J . Wong" <djwong@kernel.org>,
        "Matthew Wilcox" <willy@infradead.org>,
        "Michal Hocko" <mhocko@suse.com>,
        "Dave Chinner" <david@fromorbit.com>,
        "Rik van Riel" <riel@surriel.com>,
        "Vlastimil Babka" <vbabka@suse.cz>,
        "Johannes Weiner" <hannes@cmpxchg.org>,
        "Jonathan Corbet" <corbet@lwn.net>,
        "Linux-MM" <linux-mm@kvack.org>,
        "Linux-fsdevel" <linux-fsdevel@vger.kernel.org>,
        "LKML" <linux-kernel@vger.kernel.org>,
        "Mel Gorman" <mgorman@techsingularity.net>
Subject: Re: [PATCH 6/8] mm/vmscan: Centralise timeout values for reclaim_throttle
In-reply-to: <20211019090108.25501-7-mgorman@techsingularity.net>
References: <20211019090108.25501-1-mgorman@techsingularity.net>,
 <20211019090108.25501-7-mgorman@techsingularity.net>
Date:   Fri, 22 Oct 2021 12:06:13 +1100
Message-id: <163486477387.17149.7808824931340167601@noble.neil.brown.name>
Precedence: bulk

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