linux-kernel - Re: [PATCH] sched: sched_clock() clocksource handling.

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Message-Id: <1243929603.23657.5662.camel@twins>
Date:	Tue, 02 Jun 2009 10:00:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mundt <lethal@...ux-sh.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Daniel Walker <dwalker@...o99.com>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	John Stultz <johnstul@...ux.vnet.ibm.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk, linux-sh@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: sched_clock() clocksource handling.

On Tue, 2009-06-02 at 16:54 +0900, Paul Mundt wrote:
> On Tue, Jun 02, 2009 at 09:41:35AM +0200, Peter Zijlstra wrote:
> > On Tue, 2009-06-02 at 16:35 +0900, Paul Mundt wrote:
> > > 
> > > We already do via select_clocksource(), if we are unregistering the
> > > current one then a new one with the flag set is selected. Before that,
> > > the override is likewise given preference, and we fall back on jiffies if
> > > there is nothing else. I suppose we could try and find the "best" one,
> > > but I think the override and manual clocksource selection should be fine
> > > for this.
> > 
> > Ah, ok. So unregister calls select_clocksource again? That does leave us
> > a small window with jiffies, but I guess that's ok.
> > 
> A synchronize_rcu() would fix that up, but I think a small window with
> jiffies is less painful than sorting out RCU ordering and synchronization
> for a corner case of a corner case ;-)
> 
> > > Now that you mention it though, the sched_clocksource() assignment within
> > > select_clocksource() happens underneath the clocksource_lock, but is not
> > > using rcu_assign_pointer().
> > 
> > Right, that would want fixing indeed.
> > 
> > >  If the assignment there needs to use
> > > rcu_assign_pointer() then presumably all of the unlock paths that do
> > > select_clocksource() will have to synchronize_rcu()?
> > 
> > No, you only have to do sync_rcu() when stuff that could have referenced
> > is going away and you cannot use call_rcu().
> > 
> > So when selecting a new clocksource, you don't need synchonization
> > because stuff doesn't go away (I think :-)
> 
> Ok, that keeps things more simplified then. How does this look?


Looks fine to me,

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
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