linux-kernel - Re: [PATCH v3 05/46] perf/x86/intel/cmt: add per-package locks

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Date:   Fri, 11 Nov 2016 10:41:07 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     David Carrillo-Cisneros <davidcc@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Vegard Nossum <vegard.nossum@...il.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Nilay Vaish <nilayvaish@...il.com>,
        Borislav Petkov <bp@...e.de>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Paul Turner <pjt@...gle.com>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v3 05/46] perf/x86/intel/cmt: add per-package locks

On Fri, 11 Nov 2016, Peter Zijlstra wrote:
> Well, its very hard to suggest alternatives, because there simply isn't
> anything of content here. This patch just adds locks, and the next few
> patches don't describe much either.
> 
> Why can't this be RCU?

AFAICT from looking at later patches the context switch path can do RMID
borrowing/stealing whatever things which need protection of the package
data. Other pathes (setup/rotation/ ...) take all the package locks to
prevent concurrent RMID operations.

I still have not figured out why all of this is necessary and unfortunately
there is no real coherent epxlanation of the overall design. The cover
letter is not really helpful either.

Thanks,

	tglx