linux-kernel - Re: [RFC PATCH v4] sched: Fix performance regression introduced by mm

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Message-ID: <20230412143934.GB162902@ziqianlu-desk2>
Date:   Wed, 12 Apr 2023 22:39:34 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        <linux-kernel@...r.kernel.org>, Olivier Dion <odion@...icios.com>,
        <michael.christie@...cle.com>
Subject: Re: [RFC PATCH v4] sched: Fix performance regression introduced by
 mm_cid

On Wed, Apr 12, 2023 at 04:26:16PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 12, 2023 at 07:42:40PM +0800, Aaron Lu wrote:
> 
> > I *guess* you might be able to see some contention with hackbench on
> > that HSW-EX system with v4.
> 
> Indeed! Notably it seems to be the wakeup from idle that trips it
> hardest:

Could it because for idle cpus, the per-cpu/mm cid is no longer valid for
the sake of compacting and when task wakes there, it will have to
re-allocate a new cid through mm_get_cid() which needs to acquire
mm->cid_lock?

> 11.31%     0.03%  swapper          [kernel.vmlinux]            [k] schedule_idle
>         |
>         --11.28%--schedule_idle
>                   |
>                   --11.27%--__schedule
>                             |
>                             |--8.61%--mm_cid_get
>                             |          |
>                             |           --5.78%--_raw_spin_lock
>                             |                     |
>                             |                      --5.56%--native_queued_spin_lock_slowpath
>                             |
>                             |--0.81%--finish_task_switch.isra.0
>                             |          |
>                             |           --0.69%--asm_sysvec_call_function_single
>                             |                     |
>                             |                      --0.58%--sysvec_call_function_single
>                             |
>                             --0.57%--switch_mm_irqs_off
>                     
>                     
>