linux-kernel - Re: [PATCH RFC tip/core/rcu 2/9] rcu: Provide cond_resched_rcu_qs() to force quiescent states in long loops

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Message-ID: <20140729175552.GX11241@linux.vnet.ibm.com>
Date:	Tue, 29 Jul 2014 10:55:52 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 2/9] rcu: Provide cond_resched_rcu_qs()
 to force quiescent states in long loops

On Tue, Jul 29, 2014 at 07:37:33PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 29, 2014 at 07:36:00PM +0200, Peter Zijlstra wrote:
> > On Tue, Jul 29, 2014 at 10:33:18AM -0700, Paul E. McKenney wrote:
> > > > No, but why can't we make the regular cond_resched() do this?
> > > 
> > > Well, I got a lot of grief when I tried it a few weeks ago.
> > > 
> > > But from what I can see, you are the maintainer or cond_resched(), so
> > > if you are good with making the normal cond_resched() do this, I am
> > > more than happy to make it so!  ;-)
> > 
> > Well, its the 'obvious' thing to do. But clearly I haven't tried so I'm
> > blissfully unaware of any problems. And the Changelog didn't inform me
> > either (you had a link in there, which I didn't read :-)
> 
> Then again, last time we touched cond_resched() we had a scalability
> issue or somesuch, or am I misremembering things?

More overhead than scalability, but yes.  That said, that was a much
heavier weight touch.  A later version with only an access to per-CPU
variable turned out to have overhead below what could be measured.
But I am comfortable with the current approach that does not touch
cond_resched() as well.

							Thanx, Paul

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