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
