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 -- 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/
