linux-kernel - Re: [PATCH 8/9] hrtimer: Allow hrtimer::function() to free the timer

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Date:	Thu, 4 Jun 2015 14:37:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, umgwanakikbuti@...il.com,
	mingo@...e.hu, ktkhai@...allels.com, rostedt@...dmis.org,
	juri.lelli@...il.com, pang.xunlei@...aro.org, oleg@...hat.com,
	wanpeng.li@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] hrtimer: Allow hrtimer::function() to free the timer


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Thu, Jun 04, 2015 at 07:59:30AM +0200, Ingo Molnar wrote:
> 
> > > --- a/include/linux/hrtimer.h
> > > +++ b/include/linux/hrtimer.h
> > > @@ -123,8 +123,10 @@ struct hrtimer_sleeper {
> > >  
> > >  #ifdef CONFIG_64BIT
> > >  # define HRTIMER_CLOCK_BASE_ALIGN	64
> > > +# define __timer_base_running(timer)	timer->base->running
> > >  #else
> > >  # define HRTIMER_CLOCK_BASE_ALIGN	32
> > > +# define __timer_base_running(timer)	timer->base->cpu_base->running
> > >  #endif
> > 
> > Please put it into the cpu_base on 64-bit as well: the base pointer is available 
> > already on 64-bit so there should be no measurable performance difference, and 
> > readability is a primary concern with all this code.
> 
> That's an extra pointer chase for no reason :-(

Only if we otherwise don't dereference cpu_base - is that the case in the relevant 
code paths?

If we already dereference cpu_base (say for the lock) and have its value loaded 
then it's totally equivalent to chasing down it in base->.

Thanks,

	Ingo
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