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
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Ingo Molnar <mi...@redhat.com>
Cc: Arnaldo Carvalho de Melo <a...@kernel.org>
Cc: Alexander Shishkin <alexander.shish...@linux.intel.com>
Cc: Jiri Olsa <jo...@redhat.com>
Cc: Alexei Starovoitov <a...@kernel.org>
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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