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

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Date:	Sat, 21 Feb 2015 15:12:01 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"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, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com,
	bobby prani <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu 0/4] Programmatic nestable expedited grace
 periods

----- Original Message -----
> 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"
> <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" <bobby.prani@...il.com>
> Sent: Saturday, February 21, 2015 1:04:28 AM
> 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?

The information about begin/end of synchronize_rcu, as well as begin/end
of rcu_barrier() seems to be very relevant here. This should perhaps be
covered tracepoints ? Isn't it already ?

Thanks,

Mathieu

> 
> - Josh Triplett
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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