2022-09-22 19:22:54

by Rafael J. Wysocki

[permalink] [raw]
Subject: Re: [PATCH] ACPI: processor idle: Practically limit "Dummy wait" workaround to old Intel systems

On 9/22/2022 8:47 PM, Dave Hansen wrote:
> Old, circa 2002 chipsets have a bug: they don't go idle when they are
> supposed to. So, a workaround was added to slow the CPU down and
> ensure that the CPU waits a bit for the chipset to actually go idle.
> This workaround is ancient and has been in place in some form since
> the original kernel ACPI implementation.
>
> But, this workaround is very painful on modern systems. The "inl()"
> can take thousands of cycles (see Link: for some more detailed
> numbers and some fun kernel archaeology).
>
> First and foremost, modern systems should not be using this code.
> Typical Intel systems have not used it in over a decade because it is
> horribly inferior to MWAIT-based idle.
>
> Despite this, people do seem to be tripping over this workaround on
> AMD system today.
>
> Limit the "dummy wait" workaround to Intel systems. Keep Modern AMD
> systems from tripping over the workaround. Remotely modern Intel
> systems use intel_idle instead of this code and will, in practice,
> remain unaffected by the dummy wait.
>
> Signed-off-by: Dave Hansen <[email protected]>
> Cc: Len Brown <[email protected]>
> Cc: Mario Limonciello <[email protected]>
> Cc: Peter Zijlstra (Intel) <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Suggested-by: Rafael J. Wysocki <[email protected]>
> Reported-by: K Prateek Nayak <[email protected]>
> Link: https://lore.kernel.org/all/[email protected]/

Acked-by: Rafael J. Wysocki <[email protected]>

or do you want me to pick this up?


> ---
> drivers/acpi/processor_idle.c | 23 ++++++++++++++++++++---
> 1 file changed, 20 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 16a1663d02d4..9f40917c49ef 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -531,10 +531,27 @@ static void wait_for_freeze(void)
> /* No delay is needed if we are in guest */
> if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
> return;
> + /*
> + * Modern (>=Nehalem) Intel systems use ACPI via intel_idle,
> + * not this code. Assume that any Intel systems using this
> + * are ancient and may need the dummy wait. This also assumes
> + * that the motivating chipset issue was Intel-only.
> + */
> + if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
> + return;
> #endif
> - /* Dummy wait op - must do something useless after P_LVL2 read
> - because chipsets cannot guarantee that STPCLK# signal
> - gets asserted in time to freeze execution properly. */
> + /*
> + * Dummy wait op - must do something useless after P_LVL2 read
> + * because chipsets cannot guarantee that STPCLK# signal gets
> + * asserted in time to freeze execution properly
> + *
> + * This workaround has been in place since the original ACPI
> + * implementation was merged, circa 2002.
> + *
> + * If a profile is pointing to this instruction, please first
> + * consider moving your system to a more modern idle
> + * mechanism.
> + */
> inl(acpi_gbl_FADT.xpm_timer_block.address);
> }
>



2022-09-22 19:26:14

by Dave Hansen

[permalink] [raw]
Subject: Re: [PATCH] ACPI: processor idle: Practically limit "Dummy wait" workaround to old Intel systems

On 9/22/22 11:53, Rafael J. Wysocki wrote:
> Acked-by: Rafael J. Wysocki <[email protected]>
>
> or do you want me to pick this up?

I'll just stick it in x86/urgent.

It's modifying code in a x86 #ifdef. I'll call it a small enclave of
sovereign x86 territory in ACPI land, just like an embassy. ;)

2022-09-23 18:55:39

by Kim Phillips

[permalink] [raw]
Subject: Re: [PATCH] ACPI: processor idle: Practically limit "Dummy wait" workaround to old Intel systems

On 9/22/22 2:01 PM, Dave Hansen wrote:
> On 9/22/22 11:53, Rafael J. Wysocki wrote:
>> Acked-by: Rafael J. Wysocki <[email protected]>
>>
>> or do you want me to pick this up?
>
> I'll just stick it in x86/urgent.
>
> It's modifying code in a x86 #ifdef. I'll call it a small enclave of
> sovereign x86 territory in ACPI land, just like an embassy. ;)

Can it be cc:[email protected], since it applies cleanly as far
back as this v5.4 commit?:

commit fa583f71a99c85e52781ed877c82c8757437b680

Author: Yin Fengwei <[email protected]>

Date: Thu Oct 24 15:04:20 2019 +0800



ACPI: processor_idle: Skip dummy wait if kernel is in guest


Thanks,

Kim

2022-09-26 21:58:38

by Dave Hansen

[permalink] [raw]
Subject: Re: [PATCH] ACPI: processor idle: Practically limit "Dummy wait" workaround to old Intel systems

On 9/23/22 11:36, Kim Phillips wrote:
> Can it be cc:[email protected], since it applies cleanly as far
> back as this v5.4 commit?:

I just sent the pull request to Linus for this fix. I realized that I
didn't tag it for [email protected] If it gets applied, I'll send a request for
it to be picked up for [email protected], via "Option 2":

> Option 2
> ********
>
> After the patch has been merged to Linus' tree, send an email to
> [email protected] containing the subject of the patch, the commit ID,
> why you think it should be applied, and what kernel version you wish it to
> be applied to.

Sorry about that.