linux-kernel - Re: [PATCH] ksoftirqd: Enable IRQs and call cond

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Date:	Tue, 20 Jan 2015 21:10:08 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Calvin Owens <calvinowens@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] ksoftirqd: Enable IRQs and call cond_resched() before
 poking RCU

On Wed, Jan 21, 2015 at 04:40:39AM +0100, Mike Galbraith wrote:
> 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)

Yeah, you do have to have a fairly violent network-based DoS attack
to see the difference.  Robert Olsson was the first to make this happen
back in the day.

							Thanx, Paul

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