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

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Date:	Thu, 4 Jun 2015 07:59:30 +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 Wed, Jun 03, 2015 at 07:41:43PM +0200, Thomas Gleixner wrote:
> > On Wed, 3 Jun 2015, Peter Zijlstra wrote:
> > >  /**
> > >   * struct hrtimer - the basic hrtimer structure
> > > @@ -153,6 +144,7 @@ struct hrtimer_clock_base {
> > >  	struct timerqueue_head	active;
> > >  	ktime_t			(*get_time)(void);
> > >  	ktime_t			offset;
> > > +	struct hrtimer		*running;
> > 
> > Aside of lacking a KernelDoc comment, it expands the struct size on
> > 32bit from 32 bytes to 36 bytes which undoes some of the recent cache
> > line optimizations I did. Mooo!
> > 
> > So we might think about storing the running timer pointer in cpu_base
> > instead for 32bit, which increases the foot print of the migration
> > base and the extra cost for the additional indirection, but it would
> > keep cache line tight for the hot pathes.
> 
> A wee something like this then?
> 
> ---
> --- 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.

Thanks,

	Ingo
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