linux-kernel - Re: [PATCH 01/35] ACPI/acpi_pad: Remove the local apic nonsense

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Date:	Wed, 18 Feb 2015 07:16:58 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, tglx@...utronix.de,
	Len Brown <lenb@...nel.org>, linux-acpi@...r.kernel.org
Subject: Re: [PATCH 01/35] ACPI/acpi_pad: Remove the local apic nonsense

On Monday, February 16, 2015 07:54:12 PM Rafael J. Wysocki wrote:
> On Monday, February 16, 2015 01:14:36 PM Peter Zijlstra wrote:
> > From: Thomas Gleixner <tglx@...utronix.de>
> > 
> > While looking through the (ab)use of the clockevents_notify() function
> > I stumbled over the following gem in the acpi_pad code:
> > 
> >   if (lapic_detected_unstable && !lapic_marked_unstable) {
> >      /* LAPIC could halt in idle, so notify users */
> >      for_each_online_cpu(i)
> >        clockevents_notify(CLOCK_EVT_NOTIFY_BROADCAST_ON, &i);
> >      lapic_marked_unstable = 1;
> >   }
> > 
> > This code calls on the cpu which detects the lapic unstable condition
> > first clockevents_notify() to tell the core code that the broadcast
> > should be enabled on all online cpus. Brilliant stuff that as it
> > notifies the core code a num_online_cpus() times that the broadcast
> > should be enabled on the current cpu.
> > 
> > This probably has never been noticed because that code got never
> > tested with NOHZ=n and HIGHRES_TIMER=n or it just worked by chance
> > because one of the other mechanisms told the core in the right way
> > that the local apic timer is wreckaged.
> > 
> > Sigh, this is:
> > 
> >  - The 4th incarnation of idle drivers which has their own mechanism
> >    to detect and deal with X86_FEATURE_ARAT.
> > 
> >  - The 2nd incarnation of fake idle mechanisms with a different set of
> >    brainmelting bugs.
> > 
> >  - Has been merged against an explicit NAK of the scheduler
> >    maintainer with the promise to improve it over time.
> > 
> >  - Another example of featuritis driven trainwreck engineering.
> > 
> >  - Another pointless waste of my time.
> > 
> > Fix this nonsense by removing that lapic detection and notification
> > logic and simply call into the clockevents code unconditonally. The
> > ARAT feature is marked in the lapic clockevent already so the core
> > code will just ignore the requests and return.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> 
> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Or I can apply it right away if you want me to.

Rafael

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