linux-kernel - Re: [RFC] dynticks: dynticks_idle is only modified locally use this

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Message-ID: <20140903171409.GZ5001@linux.vnet.ibm.com>
Date:	Wed, 3 Sep 2014 10:14:09 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] dynticks: dynticks_idle is only modified locally use
 this_cpu ops

On Wed, Sep 03, 2014 at 10:51:13AM -0500, Christoph Lameter wrote:
> On Wed, 3 Sep 2014, Paul E. McKenney wrote:
> 
> > You would prefer that I instead allocated an NR_CPUS-sized array?
> 
> Well, a shared data structure would be cleaner in general but there are
> certainly other approaches.

Per-CPU variables -are- a shared data structure.

> But lets focus on the dynticks_idle case we are discussing here rather
> than tackle the more difficult other atomics. What is checked in the loop
> over the remote cpus is the dynticks_idle value plus
> dynticks_idle_jiffies. So it seems that memory ordering is only used to
> ensure that the jiffies are seen correctly.
> 
> In that case both the dynticks_idle and dynticks_idle_jiffies could be
> placed in one 64 bit value. If this is stored and retrieved as one then
> there is no issue with ordering anymore and the barriers would no longer
> be needed.

If there was an upper bound on the propagation of values through a system,
I could buy this.

But Mike Galbraith checked the overhead of ->dynticks_idle and found
it to be too small to measure.  So doesn't seem to be a problem worth
extraordinary efforts, especially given that many systems can avoid
it simply by leaving CONFIG_NO_HZ_SYSIDLE=n.

							Thanx, Paul

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