Re: [PATCH][RFC] PM / s2idle: Clear _TIF_POLLING_NRFLAG before suspend to idle

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 16, 2020 at 01:36:11AM +0800, Chen Yu wrote:
> Suspend to idle was found to not work on Goldmont CPU recently.
> And the issue was triggered due to:
> 
> 1. On Goldmont the CPU in idle can only be woken up via IPIs,
>    not POLL mode:
>    Commit 08e237fa56a1 ("x86/cpu: Add workaround for MONITOR
>    instruction erratum on Goldmont based CPUs")
> 2. When the CPU is entering suspend to idle process, the
>   _TIF_POLLING_NRFLAG is kept on.
> 3. Commit b2a02fc43a1f ("smp: Optimize send_call_function_single_ipi()")
>    makes use of _TIF_POLLING_NRFLAG to avoid sending IPIs to
>    idle CPUs.
> 4. As a result, some IPIs related functions might not work
>    well during suspend to idle on Goldmont. For example, one
>    suspected victim:
>    tick_unfreeze() -> timekeeping_resume() -> hrtimers_resume()
>    -> clock_was_set() -> on_each_cpu() might wait forever,
>    because the IPIs will not be sent to the CPUs which are
>    sleeping with _TIF_POLLING_NRFLAG set, and Goldmont CPU
>    could not be woken up by only setting _TIF_NEED_RESCHED
>    on the monitor address.

*sigh*... just what we need.


> @@ -186,8 +187,10 @@ int cpuidle_enter_s2idle(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>  	 * be frozen safely.
>  	 */
>  	index = find_deepest_state(drv, dev, U64_MAX, 0, true);
> -	if (index > 0)
> +	if (index > 0) {
> +		__current_clr_polling();
>  		enter_s2idle_proper(drv, dev, index);
> +	}
>  
>  	return index;
>  }

So how is that commit 08e237fa56a1 not suffient? That makes
mwait_idle_with_hints() DTRT for this 'functionally challenged' piece of
hardware.

AFAICT intel_enter_s2idle() uses mwait_idle_with_hints().

What am I missing?