linux-kernel - Re: [PATCH v9 00/13] Don't compute events that won't be used in a metric.

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Date:   Wed, 29 Sep 2021 16:19:00 +0100
From:   John Garry <john.garry@...wei.com>
To:     Ian Rogers <irogers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        "Namhyung Kim" <namhyung@...nel.org>,
        <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...ux.intel.com>,
        Jin Yao <yao.jin@...ux.intel.com>, Paul Clarke <pc@...ibm.com>,
        kajoljain <kjain@...ux.ibm.com>,
        <linux-perf-users@...r.kernel.org>
CC:     Stephane Eranian <eranian@...gle.com>,
        Sandeep Dasgupta <sdasgup@...gle.com>
Subject: Re: [PATCH v9 00/13] Don't compute events that won't be used in a
 metric.

On 23/09/2021 08:46, Ian Rogers wrote:
> For a metric like:
>    EVENT1 if #smt_on else EVENT2
>      
> currently EVENT1 and EVENT2 will be measured and then when the metric
> is reported EVENT1 or EVENT2 will be printed depending on the value
> from smt_on() during the expr parsing. Computing both events is
> unnecessary and can lead to multiplexing as discussed in this thread:
> https://lore.kernel.org/lkml/20201110100346.2527031-1-irogers@google.com/
> 
> This change modifies expression parsing so that constants are
> considered when building the set of ids (events) and only events not
> contributing to a constant value are measured.

Based on some testing on my arm64 platform, no regression seen, so:

Tested-by: John Garry <john.garry@...wei.com>