linux-kernel - Re: [PATCH v2 1/3] hrtimer: add hrtimer_init

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Date:	Wed, 12 Sep 2012 16:38:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	"Yan, Zheng" <zheng.z.yan@...el.com>,
	Robert Richter <robert.richter@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 1/3] hrtimer: add hrtimer_init_cpu()

On Wed, 2012-09-12 at 16:33 +0200, Stephane Eranian wrote:

> 
> If I do:
>   for_each_possible_cpu(cpu) {
>        cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
>        hr = &cpuctx->hrtimer;
>        hrtimer_init(hr)
> }
> I don't understand why I would have to refer to per-cpu data
> (hrtimer_bases) from
> a CPU that is not equal to "cpu" here. Unless you're telling me it's
> read-only data.

No its not read only, but it is unused until you do *hrtimer_start*(),
which will test and fix.

> But still if it's per-cpu why not initialize with the correct CPU from
> the start?

To keep the interface simpler I guess. There's no great harm in your
proposal, but it is strictly speaking superfluous. I'm not sure the max
one time avoidance of a base swizzle is worth the extra interface, I'll
leave that up to Thomas.

Also, what Eric said ;-)
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