linux-kernel - Re: [PATCH RFC] pm_qos

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Message-Id: <1219682129.8515.81.camel@twins>
Date:	Mon, 25 Aug 2008 18:35:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	mgross@...ux.intel.com
Cc:	John Kacur <jkacur@...il.com>, LKML <linux-kernel@...r.kernel.org>,
	rt-users <linux-rt-users@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	arjan <arjan@...radead.org>
Subject: Re: [PATCH RFC] pm_qos_requirement might sleep

On Mon, 2008-08-25 at 09:34 -0700, mark gross wrote:
> On Fri, Aug 15, 2008 at 12:51:11AM +0200, John Kacur wrote:
> > On Thu, Aug 14, 2008 at 7:48 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > On Thu, 2008-08-14 at 08:52 -0700, mark gross wrote:
> > >
> > >> Keeping a lock around the different "target_value"s may not be so
> > >> important.  Its just a 32bit scaler value, and perhaps we can make it an
> > >> atomic type?  That way we loose the raw_spinlock.
> > >
> > > My suggestion was to keep the locking for the write side - so as to
> > > avoid stuff stomping on one another, but drop the read side as:
> > >
> > >  spin_lock
> > >  foo = var;
> > >  spin_unlock
> > >  return foo;
> > >
> > > is kinda useless, it doesn't actually serialize against the usage of
> > > foo, that is, once it gets used, var might already have acquired a new
> > > value.
> > >
> > > The only thing it would protect is reading var, but since that is a
> > > machine sized read, its atomic anyway (assuming its naturally aligned).
> > >
> > > So no need for atomic_t (its read-side is just a read too), just drop
> > > the whole lock usage from pq_qos_requirement().
> > >
> > 
> > Thanks Peter.
> > 
> > Mark, is the following patch ok with you? This should be applied to
> > mainline, and then after that no special patches are necessary for
> > real-time.
> 
> I've been thinking about this patch and I worry that the readability
> from making the use of this lock asymmetric WRT reads and writes to the
> storage address is bothersome.
> 
> I would rather make the variable an atomic.  What do you think about
> that?

It would make the write side more expensive, as we already have the two
atomic operations for the lock and unlock, this would add a third.

Then again, I doubt that this is really a fast path.

OTOH, a simple comment could clarify the situation for the reader.

Up to you I guess ;-)

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