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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jun 20, 2014 at 04:52:15PM -0700, josh@joshtriplett.org wrote:
> On Fri, Jun 20, 2014 at 04:30:33PM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 20, 2014 at 03:39:51PM -0700, josh@joshtriplett.org wrote:
> > > 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.
> > 
> > OK.  But if we are in a !PREEMPT kernel,
> 
> That's not the case I was suggesting.

Fair enough, but we still need to support !PREEMPT kernels.

>                                        *If* the kernel is fully
> preemptible, then it makes little sense to put any code in cond_resched,
> when instead another thread can simply cause a preemption if it needs a
> quiescent state.  That has the advantage of not imposing any unnecessary
> polling on code running in the kernel.

OK.  Exactly which thread are you suggesting should cause the preemption?

> In a !PREEMPT kernel, it makes a bit more sense to have cond_resched as
> a voluntary preemption point.  But voluntary preemption points don't
> make as much sense in a kernel prepared to preempt a thread anywhere.

That does sound intuitive, but I am not yet prepared to believe that
the scheduler guys missed this trick.  There might well be some good
reason for cond_resched() doing something, though I cannot think what it
might be (something to do with preempt_enable_no_resched(), perhaps?).
We should at least ask them, although if you want to do some testing
before asking them, I of course have no objection to your doing so.

							Thanx, Paul