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

[Mel Gorman]

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.

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