linux-kernel - Re: [PATCH v2] watchdog: Prefer use "ref-cycles" for NMI watchdog

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Message-ID: <20230517073119.GA2665450@hirez.programming.kicks-ass.net>
Date:   Wed, 17 May 2023 09:31:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <song@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] watchdog: Prefer use "ref-cycles" for NMI watchdog

On Tue, May 16, 2023 at 04:58:17PM -0700, Song Liu wrote:
> NMI watchdog permanently consumes one hardware counters per CPU on the
> system. For systems that use many hardware counters, this causes more
> aggressive time multiplexing of perf events.
> 
> OTOH, some CPUs (mostly Intel) support "ref-cycles" event, which is rarely
> used. Try use "ref-cycles" for the watchdog, so that one more hardware
> counter is available to the user. If the CPU doesn't support "ref-cycles",
> fall back to "cycles".
> 
> The downside of this change is that users of "ref-cycles" need to disable
> nmi_watchdog.

I still utterly hate how you hardcode ref-cycles