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
