linux-kernel - Re: [PATCH RFC] pm_qos

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Date:	Fri, 15 Aug 2008 00:51:11 +0200
From:	"John Kacur" <jkacur@...il.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	mgross@...ux.intel.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 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.

Thanks

John Kacur

View attachment "pm_qos_requirement_lock.patch" of type "text/x-patch" (707 bytes)