linux-kernel - Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct check of the active list

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Date:	Mon, 13 Apr 2015 09:53:55 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, linaro-kernel@...ts.linaro.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct
 check of the active list

On Mon, 13 Apr 2015, Preeti U Murthy wrote:
> On 04/09/2015 02:48 PM, Thomas Gleixner wrote:
> > On Thu, 9 Apr 2015, Peter Zijlstra wrote:
> >> On Thu, Apr 09, 2015 at 09:20:39AM +0200, Ingo Molnar wrote:
> >>> if at least one base is active (on my fairly standard system all cpus 
> >>> have at least one active hrtimer base all the time - and many cpus 
> >>> have two bases active), then we run hrtimer_get_softirq_time(), which 
> >>> dirties the cachelines of all 4 clock bases:
> >>>
> >>>         base->clock_base[HRTIMER_BASE_REALTIME].softirq_time = xtim;
> >>>         base->clock_base[HRTIMER_BASE_MONOTONIC].softirq_time = mono;
> >>>         base->clock_base[HRTIMER_BASE_BOOTTIME].softirq_time = boot;
> >>>         base->clock_base[HRTIMER_BASE_TAI].softirq_time = tai;
> >>>
> >>> so in practice we not only touch every cacheline in every timer 
> >>> interrupt, but we _dirty_ them, even the inactive ones.
> >>>
> >>
> >> Urgh we should really _really_ kill that entire softirq mess.
> > 
> > That's the !highres part. We cannot kill that one unless we remove all
> > support for machines which do not provide hardware for highres
> > support.
> > 
> > Now the softirq_time thing is an optimization which we added back in
> > the days when hrtimer went into the tree and Roman complained about
> > the base->get_time() invocation being overkill.
> > 
> > The reasoning behing this was that low resolution systems do not need
> > accurate time for the expiry and the forwarding because everything
> > happens tick aligned.
> > 
> > So for !HIGHRES we have:
> > 
> > static inline ktime_t hrtimer_cb_get_time(struct hrtimer *timer)
> > {
> > 	return timer->base->softirq_time;
> > }
> 
> Why is this called softirq_time when the hrtimer is being serviced in
> the hard irq context ?

For historical reasons.

Thanks,

	tglx
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