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SubjectRe: [PATCH RESEND V4 0/1] perf: Add CPU hotplug support for events
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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(-)
>

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