linux-kernel - Re: [PATCH 1/2] perf/core: Adding capability to disable PMUs event multiplexing

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Message-ID: <20191106095810.GK5671@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Nov 2019 10:58:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ganapatrao Prabhakerrao Kulkarni <gkulkarni@...vell.com>
Cc:     "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will@...nel.org" <will@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "corbet@....net" <corbet@....net>,
        "gklkml16@...il.com" <gklkml16@...il.com>
Subject: Re: [PATCH 1/2] perf/core: Adding capability to disable PMUs event
 multiplexing

On Wed, Nov 06, 2019 at 10:40:32AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2019 at 01:01:40AM +0000, Ganapatrao Prabhakerrao Kulkarni wrote:
> > When PMUs are registered, perf core enables event multiplexing
> > support by default. There is no provision for PMUs to disable
> > event multiplexing, if PMUs want to disable due to unavoidable
> > circumstances like hardware errata etc.
> > 
> > Adding PMU capability flag PERF_PMU_CAP_NO_MUX_EVENTS and support
> > to allow PMUs to explicitly disable event multiplexing.
> 
> This doesn't make sense, multiplexing relies on nothing that normal
> event scheduling doesn't also rely on.
> 
> Either you can schedule different sets of events, or you cannot.

More specifically, how is a reschedule due to rotation any different
than a reschedule due to context switch?

Both cases we do a full reprogram of the PMU.