linux-kernel - Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace periods

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Date:	Fri, 20 Feb 2015 22:04:28 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	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 tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

On Fri, Feb 20, 2015 at 05:54:09PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 20, 2015 at 08:37:37AM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 20, 2015 at 10:11:07AM +0100, Peter Zijlstra wrote:
> > > Does it really make a machine boot much faster? Why are people using
> > > synchronous gp primitives if they care about speed? Should we not fix
> > > that instead?
> > 
> > The report I heard was that it provided 10-15% faster boot times.
> 
> That's not insignificant; got more details? I think we should really
> look at why people are using the sync primitives.

Paul, what do you think about adding a compile-time debug option to
synchronize_rcu() that causes it to capture the time on entry and exit
and print the duration together with the file:line of the caller?
Similar to initcall_debug, but for blocking calls to synchronize_rcu().
Put that together with initcall_debug, and you'd have a pretty good idea
of where that holds up boot.

We do want early boot to run as asynchronously as possible, and to avoid
having later bits of boot waiting on a synchronize_rcu from earlier bits
of boot.  Switching a caller over to call_rcu() doesn't actually help if
it still has to finish a grace period before it can allow later bits to
run.  Ideally, we ought to be able to work out the "depth" of boot in
grace-periods.

Has anyone wired initcall_debug up to a bootchart-like graph?

- Josh Triplett
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