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Date: Thu, 9 Apr 2015 10:58:59 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Viresh Kumar <viresh.kumar@linaro.org>, Ingo Molnar <mingo@redhat.com>,
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        Preeti U Murthy <preeti@linux.vnet.ibm.com>
Subject: Re: [PATCH] hrtimer: Replace cpu_base->active_bases with a direct
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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.

All it needs it hrtimer_start*() returning -ENOTIME when it cannot queue
the timer.

Of course, all that needs it auditing all hrtimer_start*() callsites,
which is a lot of work.