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

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Date:	Thu, 9 Apr 2015 11:56:10 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	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,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct
 check of the active list

On Thu, 9 Apr 2015, Peter Zijlstra wrote:
> On Thu, Apr 09, 2015 at 11:18:52AM +0200, 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.
> 
> Oops, sorry I mixed up the two. Doesn't take away from the fact that I'd
> like to get rid of the highres softirq fallback path.

Indeed.
 
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