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

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Thu, Apr 09, 2015 at 08:28:41AM +0200, Ingo Molnar wrote:
> > > Btw., does cpu_base->active_bases even make sense? hrtimer bases are 
> > > fundamentally percpu, and to check whether there are any pending 
> > > timers is a very simple check:
> > > 
> > > 	base->active->next != NULL
> > > 
> > 
> > Yeah, that's 3 pointer dereferences from cpu_base, iow you traded a 
> > single bit test on an already loaded word for 3 potential cacheline 
> > misses.
> 
> But the clock bases are not aligned to cachelines, and we have 4 of 
> them. So in practice when we access one, we'll load the next one 
> anyway.
> 
> Furthermore the simplification is measurable, and a fair bit of it is 
> in various fast paths. I'd rather trade a bit of a cacheline footprint 
> for less overall complexity and faster code.

Plus, look at this code in hrtimer_run_queues():

        for (index = 0; index < HRTIMER_MAX_CLOCK_BASES; index++) {
                base = &cpu_base->clock_base[index];
                if (!base->active.next)
                        continue;

                if (gettime) {
                        hrtimer_get_softirq_time(cpu_base);
                        gettime = 0;
                }

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.

So I'd strongly argue in favor of this patch series of simplification: 
it makes the code simpler and faster, and won't impact cache footprint 
in practice.

Thanks,

	Ingo
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