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

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Message-ID: <20170530194017.GN24144@tassilo.jf.intel.com>
Date:   Tue, 30 May 2017 12:40:17 -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

> > BTW there's an alternative solution in cycling the NMI watchdog over
> > all available CPUs. Then it would eventually cover all. But that's
> > less real time friendly than relying on RCU.
> 
> I don't think we need to worry too much about the watchdog being rt
> friendly. Robustness is the thing that worries me most.

Ok. Then just cycling is the most robust method, and it's very
simple.

You're right. Perhaps it's better than to rely on the RCU machinery which
seems to become ever more and more complex.

People who care extremely about latencies can just turn it off.

The main problem with the proposal is that it depends on the BIOS not 
locking the TCO watchdog. On the systems were it is locked we would
either need to continue using the PMU for the watchdog, or find some other 
watchdog source that can be programmed to be a NMI.

-Andi