linux-kernel - Re: [PATCH] Revert "ACPI: processor: idle: Only flush cache on entering C3"

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Date:   Tue, 5 Apr 2022 03:59:30 +0900
From:   Akihiko Odaki <akihiko.odaki@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH] Revert "ACPI: processor: idle: Only flush cache on
 entering C3"

On 2022/04/05 3:56, Rafael J. Wysocki wrote:
> On Mon, Apr 4, 2022 at 8:25 PM Akihiko Odaki <akihiko.odaki@...il.com> wrote:
>>
>> On 2022/04/05 3:13, Rafael J. Wysocki wrote:
>>> On Sun, Apr 3, 2022 at 8:25 AM Akihiko Odaki <akihiko.odaki@...il.com> wrote:
>>>>
>>>> This reverts commit 87ebbb8c612b1214f227ebb8f25442c6d163e802.
>>>>
>>>> ACPI processor power states can be transitioned in two distinct
>>>> situations: 1. when CPU goes idle and 2. before CPU goes offline
>>>> ("playing dead") to suspend or hibernate. Case 1 is handled by
>>>> acpi_idle_enter or acpi_idle_enter_s2idle. Case 2 is handled by
>>>> acpi_idle_play_dead.
>>>>
>>>> It is necessary to flush CPU caches in case 2 even if it is not
>>>> required to transit ACPI processor power states as CPU will go
>>>> offline soon. However, the reverted commit incorrectly removed CPU
>>>> cache flushing in such a condition.
>>>
>>> I think what you mean is that the CPU cache must always be flushed in
>>> acpi_idle_play_dead(), regardless of the target C-state that is going
>>> to be requested, because this is likely to be part of a CPU offline
>>> procedure or preparation for entering a system-wide sleep state and
>>> the stale cache contents may lead to problems going forward, for
>>> example when the CPU is taken back online.
>>>
>>> If so, I will put the above information into the patch changelog.
>>
>> I guess it is causing problems because the dirty caches will not get
>> written back and the RAM becomes stale if they are not flushed. From my
>> understanding, the CPU should have an empty cache and read back contents
>> from RAM when it is taken back online.
> 
> OK, please see if the changelog I've added to the patch is looking good:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/commit/?h=bleeding-edge&id=dfbba2518aac4204203b0697a894d3b2f80134d3

It looks good for me.

Thanks,
Akihiko Odaki