Re: [PATCH RESEND V4 0/1] perf: Add CPU hotplug support for events

[Mukesh Ojha <mojha@codeaurora.org>]

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(-)
>