linux-kernel - Re: [PATCH v3 1/1] perf: Sharing PMU counters across compatible events

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Date:   Fri, 28 Sep 2018 10:03:54 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        Tejun Heo <tj@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 1/1] perf: Sharing PMU counters across compatible
 events

Hi Song,

On 09/25/2018 03:55 AM, Song Liu wrote:
> This patch tries to enable PMU sharing. To make perf event scheduling
> fast, we use special data structures.
> 
> An array of "struct perf_event_dup" is added to the perf_event_context,
> to remember all the duplicated events under this ctx. All the events
> under this ctx has a "dup_id" pointing to its perf_event_dup. Compatible
> events under the same ctx share the same perf_event_dup. The following
> figure shows a simplified version of the data structure.
> 
>       ctx ->  perf_event_dup -> master
>                      ^
>                      |
>          perf_event /|
>                      |
>          perf_event /
> 

I've not gone through the patch in detail, but I was specifically
interested in scenarios where one perf instance is counting event
systemwide and thus other perf instance fails to count the same
event for a specific workload because that event can be counted
in one hw counter only.

Ex: https://lkml.org/lkml/2018/3/12/1011

Seems this patch does not solve this issue. Please let me know if
I'm missing anything.

Thanks,
Ravi