linux-kernel - Re: [PATCH] x86/mce/therm_throt: Handle case where throttle_active_work() is called on behalf of an offline CPU

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Date:   Mon, 24 Feb 2020 21:05:08 +0000
From:   "Pandruvada, Srinivas" <srinivas.pandruvada@...el.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "bp@...en8.de" <bp@...en8.de>
CC:     "hpa@...or.com" <hpa@...or.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "chris@...is-wilson.co.uk" <chris@...is-wilson.co.uk>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>
Subject: Re: [PATCH] x86/mce/therm_throt: Handle case where
 throttle_active_work() is called on behalf of an offline CPU

On Mon, 2020-02-24 at 20:25 +0100, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> > Which is wrong as well. Trying to "fix" it in the work queue
> > callback is
> > papering over the root cause.
> > 
> > Why is any work scheduled on an outgoing CPU after this CPU
> > executed
> > thermal_throttle_offline()?
> > 
> > When thermal_throttle_offline() is invoked the cpu bound work
> > queues are
> > still functional and thermal_throttle_offline() cancels outstanding
> > work.
> > 
> > So no, please fix the root cause not the symptom.
> 
> And if you look at thermal_throttle_online() then you'll notice that
> it
> is asymetric vs. thermal_throttle_offline().
> 
> Also you want to do cancel_delayed_work_sync() and not just
> cancel_delayed_work() because only the latter guarantees that the
> work
> is not enqueued anymore while the former does not take running or
> self
> requeueing work into account.
> 
> Something like the untested patch below.
Thanks Thomas. I am testing this patch.

-Srinivas

> 
> Thanks,
> 
>         tglx
> ---
> --- a/arch/x86/kernel/cpu/mce/therm_throt.c
> +++ b/arch/x86/kernel/cpu/mce/therm_throt.c
> @@ -487,8 +487,12 @@ static int thermal_throttle_offline(unsi
>  	struct thermal_state *state = &per_cpu(thermal_state, cpu);
>  	struct device *dev = get_cpu_device(cpu);
>  
> -	cancel_delayed_work(&state->package_throttle.therm_work);
> -	cancel_delayed_work(&state->core_throttle.therm_work);
> +	/* Mask the thermal vector before draining evtl. pending work
> */
> +	l = apic_read(APIC_LVTTHMR);
> +	apic_write(APIC_LVTTHMR, l | APIC_LVT_MASKED);
> +
> +	cancel_delayed_work_sync(&state->package_throttle.therm_work);
> +	cancel_delayed_work_sync(&state->core_throttle.therm_work);
>  
>  	state->package_throttle.rate_control_active = false;
>  	state->core_throttle.rate_control_active = false;