linux-kernel - Re: [PATCH 1/2] perf/x86/intel: enable CPU ref

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Message-ID: <20170530172208.GL24144@tassilo.jf.intel.com>
Date:   Tue, 30 May 2017 10:22:08 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Stephane Eranian <eranian@...gle.com>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        "Liang, Kan" <kan.liang@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter

> > You would only need a single one per system however, not one per CPU.
> > RCU already tracks all the CPUs, all we need is a single NMI watchdog
> > that makes sure RCU itself does not get stuck.
> > 
> > So we just have to find a single watchdog somewhere that can trigger
> > NMI.
> 
> But then you have to IPI broadcast the NMI, which is less than ideal.

Only when the watchdog times out to print the backtraces.

> 
> RCU doesn't have that problem because the quiescent state is a global
> thing. CPU progress, which is what the NMI watchdog tests, is very much
> per logical CPU though.

RCU already has a CPU stall detector. It should work (and usually
triggers before the NMI watchdog in my experience unless the
whole system is dead)

-Andi