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

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Message-id: <163226045956.21861.7998898955979000139@noble.neil.brown.name>
Date:   Wed, 22 Sep 2021 07:40:59 +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>
Subject: Re: [PATCH 1/5] mm/vmscan: Throttle reclaim until some writeback
 completes if congested

On Tue, 21 Sep 2021, 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.  An Interruptible sleep only makes sense if the "was
interrupted" status can propagate up to user-space (or to some in-kernel
handler that will clear the signal).
In particular, if reclaim_throttle() is called in a loop (which it is),
and if that loop doesn't check for signal_pending (which it doesn't),
then the next time around the loop after receiving a signal, it won't
sleep at all.  That would be bad.

In general, if you don't return an error, then you probably shouldn't
sleep Interruptible.

I notice that tasks sleep on kswapd_wait as TASK_INTERRUPTIBLE, but they
don't have any signal handling.  I suspect this isn't actually a defect
because I suspect that is it not even possible to SIGKILL kswapd.  But
the code seems misleading.  I guess I should write a patch.

Unless reclaim knows to abort completely on a signal (__GFP_KILLABLE
???) this must be an UNINTERRUPTIBLE wait.

Thanks,
NeilBrown

> 
> The original TASK_UNINTERRUPTIBLE is almost certainly a copy&paste from
> congestion_wait which may be called because a filesystem operation must
> complete before it can return to userspace so a signal waking it up is
> pointless.
> 
> -- 
> Mel Gorman
> SUSE Labs
> 
>