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Subject: Re: [PATCH RESEND V4 0/1] perf: Add CPU hotplug support for events
From:   Mukesh Ojha <mojha@codeaurora.org>
To:     lkml <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jiri Olsa <jolsa@redhat.com>, mingo@redhat.com, acme@kernel.org
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Date:   Fri, 26 Jul 2019 14:36:29 +0530
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Hi All,

Can you please review this ?

Thanks,
Mukesh

On 6/24/2019 2:31 PM, Mukesh Ojha wrote:
> Friendly ping.
>
> On 6/18/2019 7:16 PM, Mukesh Ojha wrote:
>> The embedded world, specifically Android mobile SoCs, rely on CPU
>> hotplugs to manage power and thermal constraints. These hotplugs
>> can happen at a very rapid pace. Adjacently, they also relies on
>> many perf event counters for its management. Therefore, there is
>> a need to preserve these events across hotplugs.
>>
>> In such a scenario, a perf client (kernel or user-space) can create
>> events even when the CPU is offline. If the CPU comes online during
>> the lifetime of the event, the registered event can start counting
>> spontaneously. As an extension to this, the events' count can also
>> be preserved across CPU hotplugs. This takes the burden off of the
>> clients to monitor the state of the CPU.
>>
>> The tests were conducted on arm64 device.
>> /* CPU-1 is offline: Event created when CPU1 is offline */
>>
>> / # cat /sys/devices/system/cpu/cpu1/online
>> 1
>> / # echo 0 > /sys/devices/system/cpu/cpu1/online
>>
>> Test used for testing
>> #!/bin/sh
>>
>> chmod +x *
>>
>> # Count the cycles events on cpu-1 for every 200 ms
>> ./perf stat -e cycles -I 200 -C 1 &
>>
>> # Make the CPU-1 offline and online continuously
>> while true; do
>>          sleep 2
>>          echo 0 > /sys/devices/system/cpu/cpu1/online
>>          sleep 2
>>          echo 1 > /sys/devices/system/cpu/cpu1/online
>> done
>>
>> Results:
>> / # ./test.sh
>> #           time             counts unit events
>>       0.200145885      <not counted>      cycles
>>       0.410115208      <not counted>      cycles
>>       0.619922551      <not counted>      cycles
>>       0.829904635      <not counted>      cycles
>>       1.039751614      <not counted>      cycles
>>       1.249547603      <not counted>      cycles
>>       1.459228280      <not counted>      cycles
>>       1.665606561      <not counted>      cycles
>>       1.874981926      <not counted>      cycles
>>       2.084297811      <not counted>      cycles
>>       2.293471249      <not counted>      cycles
>>       2.503231561      <not counted>      cycles
>>       2.712993332      <not counted>      cycles
>>       2.922744478      <not counted>      cycles
>>       3.132502186      <not counted>      cycles
>>       3.342255050      <not counted>      cycles
>>       3.552010102      <not counted>      cycles
>>       3.761760363      <not counted>      cycles
>>
>>      /* CPU-1 made online: Event started counting */
>>
>>       3.971459269            1925429      cycles
>>       4.181325206           19391145      cycles
>>       4.391074164             113894      cycles
>>       4.599130519            3150152      cycles
>>       4.805564737             487122      cycles
>>       5.015164581             247533      cycles
>>       5.224764529             103622      cycles
>> #           time             counts unit events
>>       5.434360831             238179      cycles
>>       5.645293799             238895      cycles
>>       5.854909320             367543      cycles
>>       6.064487966            2383428      cycles
>>
>>       /* CPU-1 made offline: counting stopped
>>
>>       6.274289476      <not counted>      cycles
>>       6.483493903      <not counted>      cycles
>>       6.693202705      <not counted>      cycles
>>       6.902956195      <not counted>      cycles
>>       7.112714268      <not counted>      cycles
>>       7.322465570      <not counted>      cycles
>>       7.532222340      <not counted>      cycles
>>       7.741975830      <not counted>      cycles
>>       7.951686246      <not counted>      cycles
>>
>>      /* CPU-1 made online: Event started counting
>>
>>       8.161469892           22040750      cycles
>>       8.371219528             114977      cycles
>>       8.580979111             259952      cycles
>>       8.790757132             444661      cycles
>>       9.000559215             248512      cycles
>>       9.210385256             246590      cycles
>>       9.420187704             243819      cycles
>>       9.630052287            7102438      cycles
>>       9.839848225             337454      cycles
>>      10.049645048             644072      cycles
>>      10.259476246            1855410      cycles
>>
>> Mukesh Ojha (1):
>>    perf: event preserve and create across cpu hotplug
>>
>>   include/linux/perf_event.h |   1 +
>>   kernel/events/core.c       | 122 
>> +++++++++++++++++++++++++++++----------------
>>   2 files changed, 79 insertions(+), 44 deletions(-)
>>