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Date: Mon, 25 Aug 2008 09:34:12 -0700 From: mark gross <mgross@...ux.intel.com> To: John Kacur <jkacur@...il.com> Cc: Peter Zijlstra <peterz@...radead.org>, 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 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? --mgross > > Thanks > > John Kacur > Subject: Remove unnecessary lock in pm_qos_requirement > > Signed-off-by: John Kacur <jkacur at gmail dot com> > > Index: linux-2.6/kernel/pm_qos_params.c > =================================================================== > --- linux-2.6.orig/kernel/pm_qos_params.c > +++ linux-2.6/kernel/pm_qos_params.c > @@ -193,14 +193,7 @@ static int find_pm_qos_object_by_minor(i > */ > int pm_qos_requirement(int pm_qos_class) > { > - int ret_val; > - unsigned long flags; > - > - spin_lock_irqsave(&pm_qos_lock, flags); > - ret_val = pm_qos_array[pm_qos_class]->target_value; > - spin_unlock_irqrestore(&pm_qos_lock, flags); > - > - return ret_val; > + return pm_qos_array[pm_qos_class]->target_value; > } > EXPORT_SYMBOL_GPL(pm_qos_requirement); > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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