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
>
>
