On Wed, Jan 21, 2015 at 10:30:07AM +0100, Thomas Gleixner wrote:
> On Tue, 20 Jan 2015, Paul E. McKenney wrote:
> > On Tue, Jan 20, 2015 at 02:21:51PM +0100, Thomas Gleixner wrote:
> > > The whole rcu_note_context_switch() in run_ksoftirqd() is silly.
> > >
> > > cond_resched()
> > > __preempt_count_add(PREEMPT_ACTIVE);
> > >
> > > __schedule();
> > > preempt_disable();
> > > rcu_note_context_switch();
> > > ....
> > >
> > > __preempt_count_sub(PREEMPT_ACTIVE);
> >
> > I agree that if should_resched() returns true as assumed above, then there
> > is no point to invoking rcu_note_context_switch(). However, the case that
> > this code applies to is when should_resched() returns false, but RCU is
> > waiting for a quiescent state from the current CPU. In that case,
> > cond_resched() won't do anything for RCU, and we do need the
> > rcu_note_context_switch().
>
> So this should be:
>
> if (!cond_resched()) {
preempt_disable();
> rcu_note_context_switch();
preempt_enable();
}
>
> Hmm?
Going forward, yes, and cond_resched_rcu_qs() in fact does something
very similar. For backporting, which is what this patch is for, we are
preserving the same double-quiescent-state behavior that existed earlier,
meaning minimal perturbation of old releases.
Seem reasonable, or do you really feel strongly about pushing this
optimization into -stable?
> > Of course, it would be better to avoid the extra RCU work in the common
> > case where cond_resched() does inovke the scheduler. And that is the
> > point of the following patch, which uses cond_resched_rcu_qs().
> > However, this use of cond_resched_rcu_qs() doesn't work in older
> > kernels. So Calvin's patch is for backporting, and the follow-up
> > patch for future kernels.
>
> I see.
Thanx, Paul
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