On Mon, Nov 07, 2022 at 01:47:23PM +0200, Ville Syrjälä wrote:
> On Wed, Nov 02, 2022 at 11:16:48PM +0100, Peter Zijlstra wrote:
> > On Wed, Nov 02, 2022 at 06:57:51PM +0200, Ville Syrjälä wrote:
> > > On Thu, Oct 27, 2022 at 06:53:23PM +0200, Peter Zijlstra wrote:
> > > > On Thu, Oct 27, 2022 at 04:09:01PM +0300, Ville Syrjälä wrote:
> > > > > On Wed, Oct 26, 2022 at 01:43:00PM +0200, Peter Zijlstra wrote:
> > > >
> > > > > > Could you please give the below a spin?
> > > > >
> > > > > Thanks. I've added this to our CI branch. I'll try to keep and eye
> > > > > on it in the coming days and let you know if anything still trips.
> > > > > And I'll report back maybe ~middle of next week if we haven't caught
> > > > > anything by then.
> > > >
> > > > Thanks!
> > >
> > > Looks like we haven't caught anything since I put the patch in.
> > > So the fix seems good.
> >
> > While writing up the Changelog, it occured to me it might be possible to
> > fix another way, could I bother you to also run the below patch for a
> > bit?
>
> I swapped in the new patch to the CI branch. I'll check back
> after a few days.
CI hasn't had anything new to report AFAICS, so looks like this
version is good as well.
>
> >
> > ---
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index cb2aa2b54c7a..daff72f00385 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4200,6 +4200,40 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> > return success;
> > }
> >
> > +static bool __task_needs_rq_lock(struct task_struct *p)
> > +{
> > + unsigned int state = READ_ONCE(p->__state);
> > +
> > + /*
> > + * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> > + * the task is blocked. Make sure to check @state since ttwu() can drop
> > + * locks at the end, see ttwu_queue_wakelist().
> > + */
> > + if (state == TASK_RUNNING || state == TASK_WAKING)
> > + return true;
> > +
> > + /*
> > + * Ensure we load p->on_rq after p->__state, otherwise it would be
> > + * possible to, falsely, observe p->on_rq == 0.
> > + *
> > + * See try_to_wake_up() for a longer comment.
> > + */
> > + smp_rmb();
> > + if (p->on_rq)
> > + return true;
> > +
> > +#ifdef CONFIG_SMP
> > + /*
> > + * Ensure the task has finished __schedule() and will not be referenced
> > + * anymore. Again, see try_to_wake_up() for a longer comment.
> > + */
> > + smp_rmb();
> > + smp_cond_load_acquire(&p->on_cpu, !VAL);
> > +#endif
> > +
> > + return false;
> > +}
> > +
> > /**
> > * task_call_func - Invoke a function on task in fixed state
> > * @p: Process for which the function is to be invoked, can be @current.
> > @@ -4217,28 +4251,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> > int task_call_func(struct task_struct *p, task_call_f func, void *arg)
> > {
> > struct rq *rq = NULL;
> > - unsigned int state;
> > struct rq_flags rf;
> > int ret;
> >
> > raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
> >
> > - state = READ_ONCE(p->__state);
> > -
> > - /*
> > - * Ensure we load p->on_rq after p->__state, otherwise it would be
> > - * possible to, falsely, observe p->on_rq == 0.
> > - *
> > - * See try_to_wake_up() for a longer comment.
> > - */
> > - smp_rmb();
> > -
> > - /*
> > - * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> > - * the task is blocked. Make sure to check @state since ttwu() can drop
> > - * locks at the end, see ttwu_queue_wakelist().
> > - */
> > - if (state == TASK_RUNNING || state == TASK_WAKING || p->on_rq)
> > + if (__task_needs_rq_lock(p))
> > rq = __task_rq_lock(p, &rf);
> >
> > /*
>
> --
> Ville Syrjälä
> Intel
--
Ville Syrjälä
Intel
