Re: [PATCH v2 2/5] sched_ext: Manage the validity of scx_rq_clock

[Changwoo Min]

Hello Peter,

On 24. 12. 2. 18:58, Peter Zijlstra wrote:
> On Mon, Dec 02, 2024 at 01:38:46PM +0900, Changwoo Min wrote:
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 95e40895a519..ab8015c8cab4 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -789,6 +789,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
> >   void update_rq_clock(struct rq *rq)
> >   {
> >   	s64 delta;
> > +	u64 clock;
> >   
> >   	lockdep_assert_rq_held(rq);
> >   
> > @@ -800,11 +801,14 @@ void update_rq_clock(struct rq *rq)
> >   		SCHED_WARN_ON(rq->clock_update_flags & RQCF_UPDATED);
> >   	rq->clock_update_flags |= RQCF_UPDATED;
> >   #endif
> > +	clock = sched_clock_cpu(cpu_of(rq));
> > +	scx_rq_clock_update(rq, clock);
>
> This adds a write to a second cacheline for *everyone* always.
>
> Please don't do that.

Thank you for the suggestion! I will update the flags only when
the scx is enabled by checking it inside the
scx_rq_clock_update() and scx_rq_clock_stale() functions.

Regards,
Changwoo Min