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

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Date:   Wed, 22 Sep 2021 09:03:51 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Dave Chinner <david@...morbit.com>
Cc:     NeilBrown <neilb@...e.de>, 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>,
        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>
Subject: Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback
 completes if congested

On Wed, Sep 22, 2021 at 04:04:47PM +1000, Dave Chinner wrote:
> On Tue, Sep 21, 2021 at 11:58:31AM +0100, Mel Gorman wrote:
> > On Tue, Sep 21, 2021 at 10:13:17AM +1000, NeilBrown wrote:
> > > 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.
> > > 
> > 
> > Because from mm/ context, I saw no reason why the task *should* be
> > uninterruptible. It's waiting on other tasks to complete IO and it is not
> > protecting device state, filesystem state or anything else. If it gets
> > a signal, it's safe to wake up, particularly if that signal is KILL and
> > the context is a direct reclaimer.
> 
> I disagree. whether the sleep should be interruptable or
> not is entirely dependent on whether the caller can handle failure
> or not. If this is GFP_NOFAIL, allocation must not fail no matter
> what the context is, so signals and the like are irrelevant.
> 
> For a context that can handle allocation failure, then it makes
> sense to wake on events that will result in the allocation failing
> immediately. But if all this does is make the allocation code go
> around another retry loop sooner, then an interruptible sleep still
> doesn't make any sense at all here...
> 

Ok, between this and Neil's mail on the same topic, I'm convinced.

-- 
Mel Gorman
SUSE Labs