linux-kernel - Re: [PATCH v2 2/2] perf parse-events: Architecture specific leader override

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Date:   Fri, 19 Nov 2021 14:47:43 +0000
From:   John Garry <john.garry@...wei.com>
To:     Ian Rogers <irogers@...gle.com>, Andi Kleen <ak@...ux.intel.com>,
        "Jiri Olsa" <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
        Kajol Jain <kjain@...ux.ibm.com>,
        "Paul A . Clarke" <pc@...ibm.com>,
        "Arnaldo Carvalho de Melo" <acme@...nel.org>,
        Riccardo Mancini <rickyman7@...il.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
        <linux-perf-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Vineet Singh" <vineet.singh@...el.com>
CC:     <eranian@...gle.com>
Subject: Re: [PATCH v2 2/2] perf parse-events: Architecture specific leader
 override

On 18/11/2021 22:06, Ian Rogers wrote:
> Currently topdown events must appear after a slots event:
> 
> $ perf stat -e '{slots,topdown-fe-bound}' /bin/true
> 
>   Performance counter stats for '/bin/true':
> 
>           3,183,090      slots
>             986,133      topdown-fe-bound
> 
> Reversing the events yields:
> 
> $ perf stat -e '{topdown-fe-bound,slots}' /bin/true
> Error:
> The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (topdown-fe-bound).
> 
> For metrics the order of events is determined by iterating over a
> hashmap, and so slots isn't guaranteed to be first which can yield this
> error.
> 
> Change the set_leader in parse-events, called when a group is closed, so
> that rather than always making the first event the leader, if the slots
> event exists then it is made the leader. It is then moved to the head of
> the evlist otherwise it won't be opened in the correct order.
> 
> The result is:
> 
> $ perf stat -e '{topdown-fe-bound,slots}' /bin/true

Just curious - does this just affect topdown events? I think x86 is the 
only arch which has them.

Thanks,
John