[PATCH V2 0/1] perf: Add CPU hotplug support for events

[Mukesh Ojha]

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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