linux-kernel - Re: [PATCH 4/5] perf/x86: Change x86_pmu::limit

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Message-ID: <20220511200657.GB76023@worktop.programming.kicks-ass.net>
Date:   Wed, 11 May 2022 22:06:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     x86@...nel.org, eranian@...gle.com, linux-kernel@...r.kernel.org,
        acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org
Subject: Re: [PATCH 4/5] perf/x86: Change x86_pmu::limit_period signature

On Wed, May 11, 2022 at 01:47:06PM -0400, Liang, Kan wrote:
> > @@ -1386,19 +1387,14 @@ int x86_perf_event_set_period(struct per
> >   		hwc->last_period = period;
> >   		ret = 1;
> >   	}
> > -	/*
> > -	 * Quirk: certain CPUs dont like it if just 1 hw_event is left:
> > -	 */
> > -	if (unlikely(left < 2))
> > -		left = 2;
> > 
> 
> Is the quirk accidentally deleted?
> We should still need the quirk for certain CPUs.

No, but I did forget to write about it in the Changelog :/

IIRC it was Nehalem that triggered that and that should now be covered
by nhm_limit_period().

Or are you aware of more machines that need this?

Anyway, perhaps this should be its own separate commit.