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Date: Wed, 21 Jun 2023 17:41:03 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Swapnil Sapkal <Swapnil.Sapkal@....com>,
Peter Zijlstra <peterz@...radead.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
John Johansen <john.johansen@...onical.com>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
Aaron Lu <aaron.lu@...el.com>, x86@...nel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [tip: sched/core] sched: Fix performance regression introduced by
mm_cid
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.
https://www.efficios.com
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