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SubjectRe: [PATCH] psi: fix possible missing or delayed pending event
On Sun, Sep 18, 2022 at 10:16:53PM -0700, Suren Baghdasaryan wrote:
> On Sun, Sep 18, 2022 at 3:55 AM Hao Lee <haolee.swjtu@gmail.com> wrote:
> >
> > On Sat, Sep 17, 2022 at 09:44:12PM -0700, Suren Baghdasaryan wrote:
> > > On Sat, Sep 17, 2022 at 12:31 AM Hao Lee <haolee.swjtu@gmail.com> wrote:
> > > >
> > > > On Fri, Sep 16, 2022 at 11:08:34PM -0700, Suren Baghdasaryan wrote:
> > > > > On Wed, Sep 14, 2022 at 2:30 AM Hao Lee <haolee.swjtu@gmail.com> wrote:
> > > > > >
> > > > > > When a pending event exists and growth is less than the threshold, the
> > > > > > current logic is to skip this trigger without generating event. However,
> > > > > > from e6df4ead85d9 ("psi: fix possible trigger missing in the window"),
> > > > > > our purpose is to generate event as long as pending event exists and the
> > > > > > rate meets the limit. This patch fixes the possible pending-event
> > > > > > missing or delay.
> > > > > >
> > > > > > Fixes: e6df4ead85d9 ("psi: fix possible trigger missing in the window")
> > > > > > Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>
> > > > > > ---
> > > > > > kernel/sched/psi.c | 2 +-
> > > > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > >
> > > > > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > > > > > index 9711827e3..0bae4ee2b 100644
> > > > > > --- a/kernel/sched/psi.c
> > > > > > +++ b/kernel/sched/psi.c
> > > > > > @@ -539,7 +539,7 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> > > > > >
> > > > > > /* Calculate growth since last update */
> > > > > > growth = window_update(&t->win, now, total[t->state]);
> > > > > > - if (growth < t->threshold)
> > > > > > + if (growth < t->threshold && !t->pending_event)
> > > > >
> > > > > I'm not sure how this additional condition changes things. Current
> > > > > logic is to set t->pending_event=true whenever growth exceeds the
> > > > > t->threshold. This patch will change this logic into setting
> > > > > t->pending_event=true also when t->pending_event=true.
> > > >
> > > > This is right.
> > > >
> > > > > But why would
> > > > > you want to set t->pending_event=true if it's already true? What am I
> > > > > missing?
> > > >
> > > > If I expand this if-else branch and the pending_event statement
> > > > to a more detailed snippet, it will be like this:
> > > >
> > > > if (growth < t->threshold && !t->pending_event) // under threshold && no pending event. Skip.
> > > > continue;
> > > > else if (growth >= t->threshold) // above threshold. Try to generate event.
> > > > t->pending_event = true;
> > > > else // under threshold && have pending events. Try to generate event.
> > > > ; // pending_event is already true. do nothing
> > > >
> > > >
> > > > The original code didn't handle the `else` condition properly.
> > >
> > > The `else` condition in your code does nothing, and that's why the
> > > original code does not implement a handler for that case.
> > >
> > > > It will
> > > > skip the trigger when its growth is under the threshold, even though it
> > > > has a pending event. This patch handles this condition correctly.
> > > >
> > > > But I think assigning true to pending_event when it's already true doesn't
> > > > have other side effects, so I eliminate the `else if` branch. Maybe we'd
> > > > better make it explicit, like the above snippet? Thanks.
> > >
> > > The new code you posted is functionally the same as the original one
> > > while being more verbose and IMO less readable. Unless you can explain
> > > the problem with the original code, I don't see any reason to change
> > > it.
> >
> > Hi, for the original code, let's assume t->pending_event is true:
> > * if new_stall is false, we will try to check event ratelimit and
> > generate an event for this psi_trigger. This case is right.
> > * but if new_stall is true, we will skip this psi_trigger if growth
> > growth < t->threshold. I think we shouldn't skip this psi_trigger
> > in this case because it has a pending event.
>
> Ok, I see the issue now. I think the following fix would be the simplest:
>
> /* Calculate growth since last update */
> growth = window_update(&t->win, now, total[t->state]);
> - if (growth < t->threshold)
> - continue;
> + if (!t->pending_event) {
> + if (growth < t->threshold)
> + continue;
>
> - t->pending_event = true;
> + t->pending_event = true;
> + }

Great! I will update in v2.

>
>
> >
> > >
> > > >
> > > > >
> > > > > > continue;
> > > > > >
> > > > > > t->pending_event = true;
> > > > > > --
> > > > > > 2.21.0
> > > > > >

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