linux-kernel - Re: [PATCH] a local-timer-free version of RCU

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Date:	Wed, 10 Nov 2010 18:45:07 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Joe Korty <joe.korty@...r.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	mathieu.desnoyers@...icios.com, dhowells@...hat.com,
	loic.minier@...aro.org, dhaval.giani@...il.com, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, josh@...htriplett.org,
	houston.jim@...cast.net
Subject: Re: [PATCH] a local-timer-free version of RCU

On Wed, Nov 10, 2010 at 06:31:11PM +0100, Peter Zijlstra wrote:
> On Wed, 2010-11-10 at 16:54 +0100, Frederic Weisbecker wrote:
> > run the sched tick and if there was nothing to do
> > for some time and we are in userspace, deactivate it. 
> 
> Not for some time, immediately, have the tick track if it was useful, if
> it was not, have it stop itself, like:
> 
> tick()
> {
>  int stop = 1;
> 
>  if (nr_running > 1)
>   stop = 0;
> 
>  if(rcu_needs_cpu())
>   stop = 0;
> 
>  ...
> 
> 
>  if (stop)
>   enter_nohz_mode();



If we are in userspace then yeah, let's enter nohz immediately.
But if we are in kernelspace, we should probably wait a bit before
switching to nohz, like one HZ, as rcu critical sections are quite
numerous.
We probably don't want to reprogram the timer everytime we enter
a new rcu_read_lock().

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