Messages in this thread | | | Subject | Re: [PATCH] ksoftirqd: Enable IRQs and call cond_resched() before poking RCU | From | Mike Galbraith <> | Date | Wed, 21 Jan 2015 04:40:39 +0100 |
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On Tue, 2015-01-20 at 12:30 -0800, Paul E. McKenney wrote: > On Tue, Jan 20, 2015 at 02:21:51PM +0100, Thomas Gleixner wrote:
> > > diff --git a/kernel/softirq.c b/kernel/softirq.c > > > index 501baa9..9e787d8 100644 > > > --- a/kernel/softirq.c > > > +++ b/kernel/softirq.c > > > @@ -656,9 +656,13 @@ static void run_ksoftirqd(unsigned int cpu) > > > * in the task stack here. > > > */ > > > __do_softirq(); > > > - rcu_note_context_switch(cpu); > > > local_irq_enable(); > > > cond_resched(); > > > + > > > + preempt_disable(); > > > + rcu_note_context_switch(cpu); > > > + preempt_enable(); > > > + > > > > 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().
I've been curious about this for ages, so now is a great time to bite the bullet and ask TheMan. A context switch is not far away, why do we need that quiescent state badly enough to tell what looks like a little white lie to get it immediately?
(I commented it out in an -rt kernel I was testing yesterday, beat it enthusiastically for a while, and box didn't _seem_ to notice that it was missing anything)
-Mike
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