On Fri, Jul 27, 2018 at 08:49:31AM -0700, Paul E. McKenney wrote:
> Hello, Peter,
>
> It occurred to me that it is wasteful to let resched_cpu() acquire
> ->pi_lock when doing something like resched_cpu(smp_processor_id()),
rq->lock
> and that it would be better to instead use set_tsk_need_resched(current)
> and set_preempt_need_resched().
>
> But is doing so really worthwhile? For that matter, are there some
> constraints on the use of those two functions that I am failing to
> allow for in the patch below?
> The resched_cpu() interface is quite handy, but it does acquire the
> specified CPU's runqueue lock, which does not come for free. This
> commit therefore substitutes the following when directing resched_cpu()
> at the current CPU:
>
> set_tsk_need_resched(current);
> set_preempt_need_resched();
That is only a valid substitute for resched_cpu(smp_processor_id()).
But also note how this can cause more context switches over
resched_curr() for not checking if TIF_NEED_RESCHED wasn't already set.
Something that might be more in line with
resched_curr(smp_processor_id()) would be:
preempt_disable();
if (!test_tsk_need_resched(current)) {
set_tsk_need_resched(current);
set_preempt_need_resched();
}
preempt_enable();
Where the preempt_enable() could of course instantly trigger the
reschedule if it was the outer most one.
> @@ -2674,10 +2675,12 @@ static __latent_entropy void
> rcu_process_callbacks(struct softirq_action *unused
> - resched_cpu(rdp->cpu); /* Provoke future context switch. */
> + set_tsk_need_resched(current);
> + set_preempt_need_resched();
That's not obviously correct. rdp->cpu had better be smp_processor_id().
> @@ -672,7 +672,8 @@ static void sync_rcu_exp_handler(void *unused)
> rcu_report_exp_rdp(rdp);
> } else {
> rdp->deferred_qs = true;
> - resched_cpu(rdp->cpu);
> + set_tsk_need_resched(t);
> + set_preempt_need_resched();
That only works if @t == current.
> }
> return;
> }
> - else
> - resched_cpu(rdp->cpu);
> + } else {
> + set_tsk_need_resched(t);
> + set_preempt_need_resched();
Similar...
> }
> @@ -791,8 +791,10 @@ static void rcu_flavor_check_callbacks(int user)
> if (t->rcu_read_lock_nesting > 0 ||
> (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
> /* No QS, force context switch if deferred. */
> - if (rcu_preempt_need_deferred_qs(t))
> - resched_cpu(smp_processor_id());
> + if (rcu_preempt_need_deferred_qs(t)) {
> + set_tsk_need_resched(t);
> + set_preempt_need_resched();
> + }
And another dodgy one..
