Re: [PATCH tip/core/rcu 0/5] Fix for cond_resched performance regression

[josh@joshtriplett.org]

On Fri, Jun 20, 2014 at 03:11:20PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 20, 2014 at 02:24:23PM -0700, josh@joshtriplett.org wrote:
> > On Fri, Jun 20, 2014 at 12:12:36PM -0700, Paul E. McKenney wrote:
> > > o	Make cond_resched() a no-op for PREEMPT=y.  This might well turn
> > > 	out to be a good thing, but it doesn't help give RCU the quiescent
> > > 	states that it needs.
> > 
> > What about doing this, together with letting the fqs logic poke
> > un-quiesced kernel code as needed?  That way, rather than having
> > cond_resched do any work, you have the fqs logic recognize that a
> > particular CPU has gone too long without quiescing, without disturbing
> > that CPU at all if it hasn't gone too long.
> 
> My next stop is to post the previous series, but with a couple of
> exports and one bug fix uncovered by testing thus far, but after
> another round of testing.  Then I am going to take a close look at
> this one:
> 
> o	Push the checks further into cond_resched(), so that the
> 	fastpath does the same sequence of instructions that the original
> 	did.  This might work well, but requires IPIs, which are not so
> 	good for latencies on the remote CPU.  It nevertheless might be a
> 	decent long-term solution given that if your CPU is spending many
> 	jiffies looping in the kernel, you aren't getting good latencies
> 	anyway.  It also has the benefit of allowing RCU to take advantage
> 	of the implicit quiescent states of all cond_resched() calls,
> 	and of eliminating the need for a separate cond_resched_rcu_qs()
> 	and for RCU_COND_RESCHED_QS.
> 
> The one you call out is of course interesting as well.  But there are
> a couple of questions:
> 
> 1.	Why wasn't cond_resched() a no-op in CONFIG_PREEMPT to start
> 	with?  It just seems to obvious a thing to do for it to possibly
> 	be an oversight.  (What, me paranoid?)
> 
> 2.	When RCU recognizes that a particular CPU has gone too long,
> 	exactly what are you suggesting that RCU do about it?  When
> 	formulating your answer, please give due consideration to the
> 	implications of that CPU being a NO_HZ_FULL CPU.  ;-)

Send it an IPI that either causes it to flag a quiescent state
immediately if currently quiesced or causes it to quiesce at the next
opportunity if not.

- Josh Triplett