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Date: Thu, 22 Jan 2015 11:39:39 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
cc: Calvin Owens <calvinowens@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joe Perches <joe@perches.com>, Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH] ksoftirqd: Enable IRQs and call cond_resched() before
 poking RCU
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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