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

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Message-ID: <20150222035806.GA4452@thin>
Date:	Sat, 21 Feb 2015 19:58:07 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	paulmck@...ux.vnet.ibm.com, Peter Zijlstra <peterz@...radead.org>,
	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 Sat, Feb 21, 2015 at 07:51:34AM -0800, Arjan van de Ven wrote:
> >>
> >>there's a few others as well that I'm chasing down...
> >>.. but the flip side, prior to running ring 3 code, why NOT do fast expedites?
> >
> >It would be good to have before-and-after measurements of actual
> >boot time.  Are these numbers available?
> 
> To show the boot time, I'm using the timestamp of the "Write protecting" line,
> that's pretty much the last thing we print prior to ring 3 execution.

That's a little sad; we ought to be write-protecting kernel read-only
data as *early* as possible.

> A kernel with default RCU behavior (inside KVM, only virtual devices) looks like this:
> 
> [    0.038724] Write protecting the kernel read-only data: 10240k
> 
> a kernel with expedited RCU (using the command line option, so that I don't have
> to recompile between measurements and thus am completely oranges-to-oranges)
> 
> [    0.031768] Write protecting the kernel read-only data: 10240k
> 
> which, in percentage, is an 18% improvement.

Nice improvement, but that suggests that we're spending far too much
time waiting on RCU grace periods at boot time.

- Josh Triplett
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