linux-kernel - Re: [PATCH v3 3/3] intel_idle: Provide enter

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Message-ID: <CAJZ5v0gmg_6OnzR6BNm+3Mx0Wzsf2nPQEZDutYu_-Px-TsvX-w@mail.gmail.com>
Date: Tue, 12 Nov 2024 12:28:14 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Patryk Wlazlyn <patryk.wlazlyn@...ux.intel.com>, Dave Hansen <dave.hansen@...el.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, 
	rafael.j.wysocki@...el.com, len.brown@...el.com, 
	artem.bityutskiy@...ux.intel.com, dave.hansen@...ux.intel.com
Subject: Re: [PATCH v3 3/3] intel_idle: Provide enter_dead() handler for SRF

On Tue, Nov 12, 2024 at 12:18 PM Patryk Wlazlyn
<patryk.wlazlyn@...ux.intel.com> wrote:
>
> > This series has said multiple times how the old algorithm is wrong.  But
> > it never actually _fixed_ the bad algorithm, only worked around it.
> >
> > Does mwait_play_dead() itself need to get fixed?
>
> I don't think so. The old algorithm gives fairly good heuristic for computing
> the mwait hint for the deepest cstate. Even though it's not guaranteed to work,
> it does work on most of the platforms that don't early return. I think we should
> leave it, but prefer idle_driver.

IOW, as a fallback mechanism, it is as good as it gets.

As the primary source of information though, not quite.

> >> Define the enter_dead() handler for SRF.
> >
> > This effectively gets the mwait hints from ______ instead of using the
> > calculation in mwait_play_dead().
>
> Ok.
>
> >> +static __cpuidle int intel_idle_enter_dead(struct cpuidle_device *dev,
> >> +                       int index)
> >> +{
> >> +    struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> >> +    struct cpuidle_state *state = &drv->states[index];
> >> +    unsigned long eax = flg2MWAIT(state->flags);
> >> +
> >> +    /* Retruns only in case of an error. */
> >
> >        ^ returns?
>
> Yup. Will fix.
>
>