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From: Thomas Gleixner <tglx@linutronix.de>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Kate Stewart <kstewart@linuxfoundation.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	x86@kernel.org, Ashok Raj <ashok.raj@intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	Clemens Ladisch <clemens@ladisch.de>,
	linux-kernel@vger.kernel.org,
	Stephane Eranian <eranian@google.com>,
	Ricardo Neri <ricardo.neri@intel.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	iommu@lists.linux-foundation.org, Tony Luck <tony.luck@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <andi.kleen@intel.com>, Borislav Petkov <bp@suse.de>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [RFC PATCH v4 05/21] x86/hpet: Reserve timer for the HPET hardlockup detector
Date: Fri, 14 Jun 2019 18:10:18 +0200 (CEST)
Message-ID: <alpine.DEB.2.21.1906141726190.1722@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190614011454.GA6347@ranerica-svr.sc.intel.com>

On Thu, 13 Jun 2019, Ricardo Neri wrote:

> On Tue, Jun 11, 2019 at 09:54:25PM +0200, Thomas Gleixner wrote:
> > On Thu, 23 May 2019, Ricardo Neri wrote:
> > 
> > > HPET timer 2 will be used to drive the HPET-based hardlockup detector.
> > > Reserve such timer to ensure it cannot be used by user space programs or
> > > for clock events.
> > > 
> > > When looking for MSI-capable timers for clock events, skip timer 2 if
> > > the HPET hardlockup detector is selected.
> > 
> > Why? Both the changelog and the code change lack an explanation why this
> > timer is actually touched after it got reserved for the platform. The
> > reservation should make it inaccessible for other things.
> 
> hpet_reserve_platform_timers() will give the HPET char driver a data
> structure which specifies which drivers are reserved. In this manner,
> they cannot be used by applications via file opens. The timer used by
> the hardlockup detector should be marked as reserved.
> 
> Also, hpet_msi_capability_lookup() populates another data structure
> which is used when obtaining an unused timer for a HPET clock event.
> The timer used by the hardlockup detector should not be included in such
> data structure.
> 
> Is this the explanation you would like to see? If yes, I will include it
> in the changelog.

Yes, the explanation makes sense. The code still sucks. Not really your
fault, but this is not making it any better.

What bothers me most is the fact that CONFIG_X86_HARDLOCKUP_DETECTOR_HPET
removes one HPET timer unconditionally. It neither checks whether the hpet
watchdog is actually enabled on the command line, nor does it validate
upfront whether the HPET supports FSB delivery.

That wastes an HPET timer unconditionally for no value. Not that I
personally care much about /dev/hpet, but some older laptops depend on HPET
per cpu timers as the local APIC timer stops in C2/3. So this unconditional
reservation will cause regressions for no reason.

The proper approach here is to:

 1) Evaluate the command line _before_ hpet_enable() is invoked

 2) Check the availability of FSB delivery in hpet_enable()

Reserve an HPET channel for the watchdog only when #1 and #2 are true.

Thanks,

	tglx


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Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-24  1:16 [RFC PATCH v4 00/21] Implement an HPET-based " Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 01/21] x86/msi: Add definition for NMI delivery mode Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 02/21] x86/hpet: Expose hpet_writel() in header Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 03/21] x86/hpet: Calculate ticks-per-second in a separate function Ricardo Neri
2019-06-14 15:54   ` Thomas Gleixner
2019-06-14 15:59     ` Thomas Gleixner
2019-06-18 22:48     ` Ricardo Neri
2019-06-18 23:13       ` Thomas Gleixner
2019-05-24  1:16 ` [RFC PATCH v4 04/21] x86/hpet: Add hpet_set_comparator() for periodic and one-shot modes Ricardo Neri
2019-06-14 18:17   ` Thomas Gleixner
2019-06-18 22:48     ` Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 05/21] x86/hpet: Reserve timer for the HPET hardlockup detector Ricardo Neri
2019-06-11 19:54   ` Thomas Gleixner
2019-06-14  1:14     ` Ricardo Neri
2019-06-14 16:10       ` Thomas Gleixner [this message]
2019-06-18 22:48         ` Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 06/21] x86/hpet: Configure the timer used by the " Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 07/21] watchdog/hardlockup: Define a generic function to detect hardlockups Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 08/21] watchdog/hardlockup: Decouple the hardlockup detector from perf Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 09/21] x86/nmi: Add a NMI_WATCHDOG NMI handler category Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 10/21] watchdog/hardlockup: Add function to enable NMI watchdog on all allowed CPUs at once Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 11/21] x86/watchdog/hardlockup: Add an HPET-based hardlockup detector Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 12/21] watchdog/hardlockup/hpet: Adjust timer expiration on the number of monitored CPUs Ricardo Neri
2019-06-11 20:11   ` Thomas Gleixner
2019-06-18 22:46     ` Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 13/21] x86/watchdog/hardlockup/hpet: Determine if HPET timer caused NMI Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 14/21] watchdog/hardlockup: Use parse_option_str() to handle "nmi_watchdog" Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 15/21] watchdog/hardlockup/hpet: Only enable the HPET watchdog via a boot parameter Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 16/21] x86/watchdog: Add a shim hardlockup detector Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 17/21] x86/tsc: Switch to perf-based hardlockup detector if TSC become unstable Ricardo Neri
2019-06-07  0:35   ` Stephane Eranian via iommu
2019-06-07 14:14     ` Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 18/21] x86/apic: Add a parameter for the APIC delivery mode Ricardo Neri
2019-06-16  9:55   ` Thomas Gleixner
2019-06-18 22:47     ` Ricardo Neri
2019-06-18 23:15       ` Thomas Gleixner
2019-05-24  1:16 ` [RFC PATCH v4 19/21] iommu/vt-d: Rework prepare_irte() to support per-irq " Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 20/21] iommu/vt-d: hpet: Reserve an interrupt remampping table entry for watchdog Ricardo Neri
2019-06-16 18:42   ` Thomas Gleixner
2019-06-16 19:21   ` Thomas Gleixner
2019-06-17  8:25     ` Thomas Gleixner
2019-06-17 21:38       ` Stephane Eranian via iommu
2019-06-17 23:08         ` Thomas Gleixner
2019-06-19 15:43           ` Jacob Pan
2019-06-21 15:33             ` Thomas Gleixner
2019-06-21 17:31               ` Jacob Pan
2019-06-21 18:39                 ` Jacob Pan
2019-06-21 20:05                   ` Thomas Gleixner
2019-06-21 23:55                     ` Ricardo Neri
2019-06-22  7:21                       ` Thomas Gleixner
2019-06-18 22:45       ` Ricardo Neri
2019-05-24  1:16 ` [RFC PATCH v4 21/21] x86/watchdog/hardlockup/hpet: Support interrupt remapping Ricardo Neri
2019-06-16  8:44   ` Thomas Gleixner
2019-06-16  8:53     ` Thomas Gleixner

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