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())
rcu_note_context_switch();
Hmm?
> 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.
Thanks,
tglx
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
