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SubjectRe: [External] Re: [PATCH v2 2/6] perf/core: Introduce percpu perf_cgroup
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On 2022/3/22 9:01 下午, Peter Zijlstra wrote:
> On Tue, Mar 22, 2022 at 08:08:30PM +0800, Chengming Zhou wrote:
>> Although we don't have incosistency problem any more, we can
>> have other problem like:
>>
>> CPU1 CPU2
>> (in context_switch) (attach running task)
>> prev->cgroups = cgrp2
>> perf_cgroup_sched_switch(prev, next)
>> cgrp2 == cgrp2 is True
>>
>
> Again, I'm not following, how can you attach to a running task from
> another CPU ?

Hi Peter, I make a little testcase which can reproduce the race
problem, on system with PSI disabled. Because when PSI enabled,
cgroup_move_task() will hold rq lock to assign task->cgroups.

```
#!/bin/bash

cd /sys/fs/cgroup/perf_event

mkdir cg1
mkdir cg2

perf stat -e cycles --cgroup /cg1 &

cg_run()
{
cg=$1
shift
echo $BASHPID > $cg/cgroup.procs
$@
}

cg_run cg1 schbench -r 100 &
cg_run cg2 schbench -r 100 &

while true; do
for i in $(cat cg1/cgroup.procs); do
echo $i > cg2/cgroup.procs
done
for i in $(cat cg2/cgroup.procs); do
echo $i > cg1/cgroup.procs
done
done
```

Some seconds later, dmesg will show the WARNING message:

[ 51.777830] WARNING: CPU: 2 PID: 1849 at kernel/events/core.c:869 perf_cgroup_switch+0x246/0x290
[ 51.779167] Modules linked in:
[ 51.779696] CPU: 2 PID: 1849 Comm: schbench Not tainted 5.17.0-rc8 #28
[ 51.780691] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
[ 51.782353] RIP: 0010:perf_cgroup_switch+0x246/0x290
[ 51.783145] Code: 0f 0b e9 0b ff ff ff 48 83 7c 24 08 00 74 0c e8 00 7e f7 ff fb 66 0f 1f 44 00 00 48 83 c4 10 5b 5d 41 5c 41 5d 41 5e 41 5f c3 <0f> 0b e9 4f fe ff ff e8 be 7e f7 ff e8 a9 1f 93 00 89 c0 49 c7 c5
[ 51.785804] RSP: 0018:ffffba4440fcbd80 EFLAGS: 00010086
[ 51.786617] RAX: 0000000000000002 RBX: ffff8d78eb8b7200 RCX: 0000000000000000
[ 51.787696] RDX: 0000000000000000 RSI: ffffffffae1c83db RDI: ffffffffae185a69
[ 51.788777] RBP: ffff8d78eb8aad40 R08: 0000000000000001 R09: 0000000000000001
[ 51.789854] R10: 0000000000000000 R11: ffff8d78eb8b7220 R12: ffff8d78eb8b7208
[ 51.790929] R13: ffff8d78eb8aafa0 R14: ffff8d74cd6bb600 R15: 0000000000000000
[ 51.792006] FS: 00007fedaaffd700(0000) GS:ffff8d78eb880000(0000) knlGS:0000000000000000
[ 51.793223] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 51.794122] CR2: 000055e4bf2b696c CR3: 00000001128a2003 CR4: 0000000000370ee0
[ 51.795209] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 51.796292] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 51.797375] Call Trace:
[ 51.797828] <TASK>
[ 51.798229] __perf_event_task_sched_in+0x151/0x350
[ 51.799009] ? lock_release+0x1ed/0x2e0
[ 51.799640] finish_task_switch+0x1d3/0x2e0
[ 51.800328] ? __switch_to+0x136/0x4b0
[ 51.800953] __schedule+0x33e/0xae0
[ 51.801535] schedule+0x4e/0xc0
[ 51.802080] exit_to_user_mode_prepare+0x172/0x2a0
[ 51.802850] ? asm_sysvec_apic_timer_interrupt+0xa/0x20
[ 51.803675] irqentry_exit_to_user_mode+0x5/0x40
[ 51.804413] sysvec_apic_timer_interrupt+0x5c/0xd0
[ 51.805183] asm_sysvec_apic_timer_interrupt+0x12/0x20

Thanks.

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