Re: [External] Re: [PATCH v2 2/2] sched/core: Avoid double calling update_rq_clock()

[Hao Jia]

On 2023/5/10 Peter Zijlstra wrote:
> On Wed, May 10, 2023 at 04:34:50PM +0800, Hao Jia wrote:

> > For the __balance_push_cpu_stop() case, we remove update_rq_clock() from
> > the __migrate_task() function to avoid double updating the rq clock.
> > And in order to avoid missing rq clock update, add update_rq_clock()
> > call before migration_cpu_stop() calls __migrate_task().
> >
> > For the unthrottle_cfs_rq() case, we use rq_clock_start_loop_update() to
> > prevent multiple calls to update_rq_clock() in unthrottle_cfs_rq().
> >
> > Note that the rq clock has been updated before the set_rq_offline()
> > function runs, so we don't need to call update_rq_clock() in
> > unthrottle_offline_cfs_rqs().
>
> This reads like 3 separate issues -- in which case this ought to be 3
> separate patches.

I will do it in the next version.

Thanks,
Hao

> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index ec7b3e0a2b20..9c712f29e5a4 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1536,6 +1536,27 @@ static inline void rq_clock_skip_update(struct rq *rq)
> >   	rq->clock_update_flags |= RQCF_REQ_SKIP;
> >   }
> >   
> > +/*
> > + * During cpu offlining and rq wide unthrottling, we can trigger
> > + * an update_rq_clock() for several cfs and rt runqueues (Typically
> > + * when using list_for_each_entry_*)
> > + * rq_clock_start_loop_update() can be called after updating the clock
> > + * once and before iterating over the list to prevent multiple update.
> > + * After the iterative traversal, we need to call rq_clock_stop_loop_update()
> > + * to restore rq->clock_update_flags.
>           ^^^^^^^
> > + */
> > +static inline void rq_clock_start_loop_update(struct rq *rq)
> > +{
> > +	lockdep_assert_rq_held(rq);
> > +	rq->clock_update_flags |= RQCF_ACT_SKIP;
> > +}
> > +
> > +static inline void rq_clock_stop_loop_update(struct rq *rq)
> > +{
> > +	lockdep_assert_rq_held(rq);
> > +	rq->clock_update_flags &= ~RQCF_ACT_SKIP;
> > +}
>
> This does not restore the flag, it clears the flag.

Thanks for your review.
I will also modify it in the next version.

Thanks,
Hao