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

Mathieu Desnoyers
2023-06-21T21:40:52Z

On 6/21/23 14:51, Mathieu Desnoyers wrote:
> 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.

If apparmor happens to be the culprit here, we should have a hard look
at this commit:

commit df323337e50 "apparmor: Use a memory pool instead per-CPU caches"

Which turned a per-cpu cache into a global memory pool protected by a
spinlock. It may benefit RT, but it does not appear to be so great at
scaling.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.