Re: [tip: sched/core] sched: Fix performance regression introduced by mm_cid

Mathieu Desnoyers
2023-06-21T18:51:30Z

On 6/21/23 12:36, Swapnil Sapkal wrote:
> Hello Mathieu,
>
[...]
>>
>> I suspect the regression is caused by the mm_count cache line bouncing.
>>
>> Please try with this additional patch applied:
>>
>> https://lore.kernel.org/lkml/20230515143536.114960-1-mathieu.desnoyers@efficios.com/
>
> Thanks for the suggestion. I tried out with the patch you suggested. I
> am seeing
> improvement in hackbench numbers with mm_count padding. But this is not
> matching
> with what we achieved through reverting the new mm_cid patch.
>
> Below are the results on the 1 Socket 4th Generation EPYC Processor (1 x
> 96C/192T):
>
> Threads:
>
> Test:              Base (v6.4-rc1)   Base + new_mmcid_reverted  Base +
> mm_count_padding
>  1-groups:         5.23 (0.00 pct)         4.61 (11.85 pct)        5.11
> (2.29 pct)
>  2-groups:         4.99 (0.00 pct)         4.72 (5.41 pct)         5.00
> (-0.20 pct)
>  4-groups:         5.96 (0.00 pct)         4.87 (18.28 pct)        5.86
> (1.67 pct)
>  8-groups:         6.58 (0.00 pct)         5.44 (17.32 pct)        6.20
> (5.77 pct)
> 16-groups:        11.48 (0.00 pct)         8.07 (29.70 pct)       10.68
> (6.96 pct)
>
> Processes:
>
> Test:              Base (v6.4-rc1)  Base + new_mmcid_reverted   Base +
> mm_count_padding
>  1-groups:         5.19 (0.00 pct)         4.90 (5.58 pct)         5.19
> (0.00 pct)
>  2-groups:         5.44 (0.00 pct)         5.39 (0.91 pct)         5.39
> (0.91 pct)
>  4-groups:         5.69 (0.00 pct)         5.64 (0.87 pct)         5.64
> (0.87 pct)
>  8-groups:         6.08 (0.00 pct)         6.01 (1.15 pct)         6.04
> (0.65 pct)
> 16-groups:        10.87 (0.00 pct)        10.83 (0.36 pct)        10.93
> (-0.55 pct)
>
> The ibs profile shows that function __switch_to_asm() is coming at top
> in baseline
> run and is not seen with mm_count padding patch. Will be attaching full
> ibs profile
> data for all the 3 runs:
>
> # Base (v6.4-rc1)
> Threads:
> Total time: 11.486 [sec]
>
>    5.15%  sched-messaging  [kernel.vmlinux]      [k] __switch_to_asm
>    4.31%  sched-messaging  [kernel.vmlinux]      [k] copyout
>    4.29%  sched-messaging  [kernel.vmlinux]      [k]
> native_queued_spin_lock_slowpath
>    4.22%  sched-messaging  [kernel.vmlinux]      [k] copyin
>    3.92%  sched-messaging  [kernel.vmlinux]      [k]
> apparmor_file_permission
>    2.91%  sched-messaging  [kernel.vmlinux]      [k] __schedule
>    2.34%  swapper          [kernel.vmlinux]      [k] __switch_to_asm
>    2.10%  sched-messaging  [kernel.vmlinux]      [k] prepare_to_wait_event
>    2.10%  sched-messaging  [kernel.vmlinux]      [k] try_to_wake_up
>    2.07%  sched-messaging  [kernel.vmlinux]      [k]
> finish_task_switch.isra.0
>    2.00%  sched-messaging  [kernel.vmlinux]      [k] pipe_write
>    1.82%  sched-messaging  [kernel.vmlinux]      [k]
> check_preemption_disabled
>    1.73%  sched-messaging  [kernel.vmlinux]      [k]
> exit_to_user_mode_prepare
>    1.52%  sched-messaging  [kernel.vmlinux]      [k] __entry_text_start
>    1.49%  sched-messaging  [kernel.vmlinux]      [k] osq_lock
>    1.45%  sched-messaging  libc.so.6             [.] write
>    1.44%  swapper          [kernel.vmlinux]      [k] native_sched_clock
>    1.38%  sched-messaging  [kernel.vmlinux]      [k] psi_group_change
>    1.38%  sched-messaging  [kernel.vmlinux]      [k] pipe_read
>    1.37%  sched-messaging  libc.so.6             [.] read
>    1.06%  sched-messaging  [kernel.vmlinux]      [k] vfs_read
>    1.01%  swapper          [kernel.vmlinux]      [k] psi_group_change
>    1.00%  sched-messaging  [kernel.vmlinux]      [k] update_curr
>
> # Base + mm_count_padding
> Threads:
> Total time: 11.384 [sec]
>
>    4.43%  sched-messaging  [kernel.vmlinux]         [k] copyin
>    4.39%  sched-messaging  [kernel.vmlinux]         [k]
> native_queued_spin_lock_slowpath
>    4.07%  sched-messaging  [kernel.vmlinux]         [k]
> apparmor_file_permission
>    4.07%  sched-messaging  [kernel.vmlinux]         [k] copyout
>    2.49%  sched-messaging  [kernel.vmlinux]         [k] entry_SYSCALL_64
>    2.37%  sched-messaging  [kernel.vmlinux]         [k] update_cfs_group
>    2.19%  sched-messaging  [kernel.vmlinux]         [k] pipe_write
>    2.00%  sched-messaging  [kernel.vmlinux]         [k]
> check_preemption_disabled
>    1.93%  swapper          [kernel.vmlinux]         [k] update_load_avg
>    1.81%  sched-messaging  [kernel.vmlinux]         [k]
> exit_to_user_mode_prepare
>    1.69%  sched-messaging  [kernel.vmlinux]         [k] try_to_wake_up
>    1.58%  sched-messaging  libc.so.6                [.] write
>    1.53%  sched-messaging  [kernel.vmlinux]         [k] psi_group_change
>    1.50%  sched-messaging  libc.so.6                [.] read
>    1.50%  sched-messaging  [kernel.vmlinux]         [k] pipe_read
>    1.39%  sched-messaging  [kernel.vmlinux]         [k] update_load_avg
>    1.39%  sched-messaging  [kernel.vmlinux]         [k] osq_lock
>    1.30%  sched-messaging  [kernel.vmlinux]         [k] update_curr
>    1.28%  swapper          [kernel.vmlinux]         [k] psi_group_change
>    1.16%  sched-messaging  [kernel.vmlinux]         [k] vfs_read
>    1.12%  sched-messaging  [kernel.vmlinux]         [k] vfs_write
>    1.10%  sched-messaging  [kernel.vmlinux]         [k]
> entry_SYSRETQ_unsafe_stack
>    1.09%  sched-messaging  [kernel.vmlinux]         [k] __switch_to_asm
>    1.08%  sched-messaging  [kernel.vmlinux]         [k] do_syscall_64
>    1.06%  sched-messaging  [kernel.vmlinux]         [k]
> select_task_rq_fair
>    1.03%  swapper          [kernel.vmlinux]         [k] update_cfs_group
>    1.00%  swapper          [kernel.vmlinux]         [k] rb_insert_color
>
> # Base + reverted_new_mm_cid
> Threads:
> Total time: 7.847 [sec]
>
>   12.14%  sched-messaging  [kernel.vmlinux]      [k]
> native_queued_spin_lock_slowpath
>    8.86%  swapper          [kernel.vmlinux]      [k]
> native_queued_spin_lock_slowpath
>    6.13%  sched-messaging  [kernel.vmlinux]      [k] copyin
>    5.54%  sched-messaging  [kernel.vmlinux]      [k]
> apparmor_file_permission
>    3.59%  sched-messaging  [kernel.vmlinux]      [k] copyout
>    2.61%  sched-messaging  [kernel.vmlinux]      [k] osq_lock
>    2.48%  sched-messaging  [kernel.vmlinux]      [k] pipe_write
>    2.33%  sched-messaging  [kernel.vmlinux]      [k]
> exit_to_user_mode_prepare
>    2.01%  sched-messaging  [kernel.vmlinux]      [k]
> check_preemption_disabled
>    1.96%  sched-messaging  [kernel.vmlinux]      [k] __entry_text_start
>    1.91%  sched-messaging  libc.so.6             [.] write
>    1.77%  sched-messaging  libc.so.6             [.] read
>    1.64%  sched-messaging  [kernel.vmlinux]      [k] mutex_spin_on_owner
>    1.58%  sched-messaging  [kernel.vmlinux]      [k] pipe_read
>    1.52%  sched-messaging  [kernel.vmlinux]      [k] try_to_wake_up
>    1.38%  sched-messaging  [kernel.vmlinux]      [k]
> ktime_get_coarse_real_ts64
>    1.35%  sched-messaging  [kernel.vmlinux]      [k] vfs_write
>    1.28%  sched-messaging  [kernel.vmlinux]      [k]
> entry_SYSRETQ_unsafe_stack
>    1.28%  sched-messaging  [kernel.vmlinux]      [k] vfs_read
>    1.25%  sched-messaging  [kernel.vmlinux]      [k] do_syscall_64
>    1.22%  sched-messaging  [kernel.vmlinux]      [k] __fget_light
>    1.18%  sched-messaging  [kernel.vmlinux]      [k] mutex_lock
>    1.12%  sched-messaging  [kernel.vmlinux]      [k] file_update_time
>    1.04%  sched-messaging  [kernel.vmlinux]      [k] _copy_from_iter
>    1.01%  sched-messaging  [kernel.vmlinux]      [k] current_time
>
> So with the reverted new_mm_cid patch, we are seeing a lot of time being
> spent in
> native_queued_spin_lock_slowpath and yet, hackbench finishes faster.
>
> I keep further digging into this please let me know if you have any
> pointers for me.

Do you have CONFIG_SECURITY_APPARMOR=y ? Can you try without ?

I notice that apparmor_file_permission appears near the top of your
profiles, and apparmor uses an internal aa_buffers_lock spinlock,
which could possibly explain the top hits for
native_queued_spin_lock_slowpath. My current suspicion is that
the raw spinlock that was taken by "Base + reverted_new_mm_cid"
changed the contention pattern on the apparmor lock enough to
speed things up by pure accident.

Thanks,

Mathieu

>
>>
>> This patch has recently been merged into the mm tree.
>>
>> Thanks,
>>
>> Mathieu
>>
> --
> Thanks and Regards,
> Swapnil

--
Mathieu Desnoyers
EfficiOS Inc.