On Wed, 21 Jan 2015, Paul E. McKenney wrote:
> 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?
No.
Thanks,
tglx
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