linux-kernel - Re: [PATCH 2/6] cpuidle: Introduce CPUIDLE_FLAG

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Message-ID: <CAJZ5v0gEhGJCkfMn6jhF2D_4BYKJZev_Eyq5AhkTeT02_cHcXQ@mail.gmail.com>
Date: Tue, 14 Jan 2025 15:01:26 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, 
	"Rafael J . Wysocki" <rafael@...nel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>, linux-pm@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH 2/6] cpuidle: Introduce CPUIDLE_FLAG_MWAIT

On Thu, Jan 2, 2025 at 4:02 PM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> From: Peter Zijlstra <peterz@...radead.org>
>
> Provide a way to tell the cpuidle core about states monitoring
> TIF_NEED_RESCHED on the hardware level, monitor/mwait users being the
> only examples in use.
>
> This will allow cpuidle core to manage TIF_NR_POLLING on behalf of all
> kinds of TIF_NEED_RESCHED watching states while keeping a necessary
> distinction for the governors between software loops polling on
> TIF_NEED_RESCHED and hardware monitored writes to thread flags.
>
> [fweisbec: _ Initialize flag from acpi_processor_setup_cstates() instead
>              of acpi_processor_setup_lpi_states(), as the latter seem to
>              be about arm64...
>            _ Rename CPUIDLE_FLAG_NO_IPI to CPUIDLE_FLAG_MWAIT]
>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
>  drivers/acpi/processor_idle.c | 3 +++
>  drivers/idle/intel_idle.c     | 5 ++++-
>  include/linux/cpuidle.h       | 1 +
>  3 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 698897b29de2..66cb5536d91e 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -806,6 +806,9 @@ static int acpi_processor_setup_cstates(struct acpi_processor *pr)
>                 if (cx->type == ACPI_STATE_C1 || cx->type == ACPI_STATE_C2)
>                         drv->safe_state_index = count;
>
> +               if (cx->entry_method == ACPI_CSTATE_FFH)
> +                       state->flags |= CPUIDLE_FLAG_MWAIT;

FFH need not mean MWAIT in principle.

FFH in _CST means MWAIT in practice because _CST is used on x86 which
implements FFH through MWAIT, but it would be good at least to put a
comment here to explain that this code is only expected to run on x86.

Or better still, add something like acpi_arch_idle_state_flags(u8
entry_method) that will return CPUIDLE_FLAG_MWAIT for ACPI_CSTATE_FFH
and 0 otherwise and then do

    state->flags |= acpi_arch_idle_state_flags(cx->entry_method);

> +
>                 /*
>                  * Halt-induced C1 is not good for ->enter_s2idle, because it
>                  * re-enables interrupts on exit.  Moreover, C1 is generally not
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index ac4d8faa3886..d52723fbeb04 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -1787,7 +1787,8 @@ static void __init intel_idle_init_cstates_acpi(struct cpuidle_driver *drv)
>                 if (cx->type > ACPI_STATE_C1)
>                         state->target_residency *= 3;
>
> -               state->flags = MWAIT2flg(cx->address);
> +               state->flags = MWAIT2flg(cx->address) | CPUIDLE_FLAG_MWAIT;
> +
>                 if (cx->type > ACPI_STATE_C2)
>                         state->flags |= CPUIDLE_FLAG_TLB_FLUSHED;
>
> @@ -2072,6 +2073,8 @@ static bool __init intel_idle_verify_cstate(unsigned int mwait_hint)
>
>  static void state_update_enter_method(struct cpuidle_state *state, int cstate)
>  {
> +       state->flags |= CPUIDLE_FLAG_MWAIT;
> +
>         if (state->flags & CPUIDLE_FLAG_INIT_XSTATE) {
>                 /*
>                  * Combining with XSTATE with IBRS or IRQ_ENABLE flags
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index a9ee4fe55dcf..b8084617aa27 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -85,6 +85,7 @@ struct cpuidle_state {
>  #define CPUIDLE_FLAG_OFF               BIT(4) /* disable this state by default */
>  #define CPUIDLE_FLAG_TLB_FLUSHED       BIT(5) /* idle-state flushes TLBs */
>  #define CPUIDLE_FLAG_RCU_IDLE          BIT(6) /* idle-state takes care of RCU */
> +#define CPUIDLE_FLAG_MWAIT             BIT(7) /* hardware need_resched() monitoring */
>
>  struct cpuidle_device_kobj;
>  struct cpuidle_state_kobj;
> --