linux-kernel - Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ

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Date:	Mon, 15 Apr 2013 12:03:54 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing
 delay from HZ

On Fri, Apr 12, 2013 at 11:38:04PM -0700, Paul E. McKenney wrote:
> On Fri, Apr 12, 2013 at 04:54:02PM -0700, Josh Triplett wrote:
> > On Fri, Apr 12, 2013 at 04:19:13PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > 
> > > Systems with HZ=100 can have slow bootup times due to the default
> > > three-jiffy delays between quiescent-state forcing attempts.  This
> > > commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based
> > > on the value of HZ.  However, this would break very large systems that
> > > require more time between quiescent-state forcing attempts.  This
> > > commit therefore also ups the default delay by one jiffy for each
> > > 256 CPUs that might be on the system (based off of nr_cpu_ids at
> > > runtime, -not- NR_CPUS at build time).
> > > 
> > > Reported-by: Paul Mackerras <paulus@....ibm.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > Something seems very wrong if RCU regularly hits the fqs code during
> > boot; feels like there's some more straightforward solution we're
> > missing.  What causes these CPUs to fall under RCU's scrutiny during
> > boot yet not actually hit the RCU codepaths naturally?
> 
> The problem is that they are running HZ=100, so that RCU will often
> take 30-60 milliseconds per grace period.  At that point, you only
> need 16-30 grace periods to chew up a full second, so it is not all
> that hard to eat up the additional 8-12 seconds of boot time that
> they were seeing.  IIRC, UP boot was costing them 4 seconds.

I added some instrumentation, which counted 202 calls to
synchronize_sched() during boot (Fedora 17 minimal install +
development tools) with a 3.8.0 kernel on a 4-cpu KVM virtual machine
on a POWER7.  Without this patch, those 202 calls take up a total of
4.32 seconds; with it, they take up 3.6 seconds.  The kernel is
compiled with HZ=100 and NR_CPUS=1024, like the standard Fedora
kernel.

I suspect a lot of the calls are in udevd and related processes.
Interestingly there were no calls to synchronize_rcu_bh or
synchronize_sched_expedited.

Paul.
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