linux-kernel - Re: [patch V2 35/67] arm/perf: Convert to hotplug state machine

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Message-ID: <20160715161654.GF19840@leverpostej>
Date:	Fri, 15 Jul 2016 17:16:54 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Anna-Maria Gleixner <anna-maria@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, rt@...utronix.de,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>
Subject: Re: [patch V2 35/67] arm/perf: Convert to hotplug state machine

On Fri, Jul 15, 2016 at 05:26:30PM +0200, Sebastian Andrzej Siewior wrote:
> * Mark Rutland | 2016-07-15 14:08:18 [+0100]:
> 
> >Hi,
> Hi Mark,
> 
> >We may have multiple PMUs (e.g. two in big.LITTLE systems), and
> >__oprofile_cpu_pmu only contains one of these. So this conversion is not
> >correct.
> >
> >We were relying on the notifier list implicitly containing a list of
> >those PMUs. It seems like we need an explicit list here.
> >
> >We keep __oprofile_cpu_pmu around for legacy 32-bit users of OProfile
> >(on non-hetereogeneous systems), and that's all that the variable should
> >be used for.
> 
> By the time I'e been looking there was only one node in the .dts. Now
> you say it is not only possible but likely to have more than one node.

Yes. For example, arch/arm/boot/dts/vexpress-v2p-ca15_a7.dts is a
big.LITTLE system with a Cortex-A7 PMU and a Cortex-A15 PMU.

> So what we need is a list which gets extended in cpu_pmu_init() instead
> of using the global pointer we have now. Correct?

Yes, that sounds right to me, with matching removal in cpu_pmu_destroy.

Thanks,
Mark.