linux-kernel - BUG in "perf: Suppress AUX/OVERWRITE records"?

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Message-ID: <ca9bbbdfada4c02eacb5064fce5d187d0f13f4f0.camel@arm.com>
Date:   Mon, 25 Mar 2019 13:20:06 +0000
From:   Ben Gainey <Ben.Gainey@....com>
To:     "mingo@...nel.org" <mingo@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "acme@...hat.com" <acme@...hat.com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        Will Deacon <Will.Deacon@....com>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: BUG in "perf: Suppress AUX/OVERWRITE records"?

Hi all

Regarding commit 1627314fb54a33ebd23bd08f2e215eaed0f44712 "perf:
Suppress AUX/OVERWRITE records", I have found that I no longer receive
PERF_RECORD_AUX on context switch when collecting data from the arm_spe
PMU driver. This is because, on context switch, the arm_spe driver
calls perf_aux_output_end with `handle->aux_flags == 0`, failing the
test added in this commit.

This is a problem as it means when capturing data for multiple threads
(using perf_event_open) where AUX data is written to a per-cpu buffer,
I can no longer accurately attribute SPE AUX data to an individual
thread.

If I read the intent of the commit as to remove OVERWRITE AUX records,
then it seems the added if condition is incorrect and should probably
be formulated as:

    if ((handle->aux_flags & ~(u64)PERF_AUX_FLAG_OVERWRITE) || !handle-
>aux_flags)

Is this correct (and would you like a patch?), or is my use of
PERF_RECORD_AUX incorrect in this case?

Thanks

Ben

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