From: Peter Zijlstra <peterz@infradead.org>
To: Mike Turquette <mturquette@ti.com>
Cc: linux-kernel@vger.kernel.org,
linux-pm@lists.linux-foundation.org, linaro-dev@lists.linaro.org,
patches@linaro.org, gregkh@suse.de, amit.kucheria@linaro.org
Subject: Re: [PATCH v3 1/2] cpumask: introduce cpumask for hotpluggable CPUs
Date: Thu, 11 Aug 2011 22:02:37 +0200 [thread overview]
Message-ID: <1313092957.26866.7.camel@twins> (raw)
In-Reply-To: <1313092487-15858-1-git-send-email-mturquette@ti.com>
On Thu, 2011-08-11 at 12:54 -0700, Mike Turquette wrote:
> On some platforms it is possible to have some CPUs which support CPU
> hotplug and some which do not. Currently the prescence of an 'online'
> sysfs entry in userspace is adequate for applications to know that a CPU
> supports hotplug, but there is no convenient way to make the same
> determination in the kernel.
>
> To better model this relationship this patch introduces a new cpumask to
> track CPUs that support CPU hotplug operations.
>
> This new cpumask is populated at boot-time and remains static for the
> life of the machine. Bits set in the mask indicate a CPU which supports
> hotplug, but make no guarantees about whether that CPU is currently
> online or not. Likewise a cleared bit in the mask indicates either a
> CPU which cannot hotplug or a lack of a populated CPU.
>
> The purpose of this new cpumask is to aid kernel code which uses CPU to
> take CPUs online and offline. Possible uses are as a thermal event
> mitigation technique or as a power capping mechanism.
Nacked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
the kernel really shouldn't be using hotplug for this (nor should
userspace really). hot-unplugging random cpus wrecks things like
cpusets. Furthermore hotplug does way too much work to use as a simple
means to idle a cpu.
Even the availability of this mask is wrong, since that implies the
information is useful, which per the above it is not, the kernel
shouldn't care about this full-stop.
The only reason for the OS to unplug a CPU is imminent and unavoidable
hardware failure. Thermal capping is not that (and yes ACPI-4.0 is a
broken piece of shit).
prev parent reply other threads:[~2011-08-11 20:03 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-08-10 20:03 [PATCH v2 0/2] new cpumask for hotpluggable CPUs Mike Turquette
2011-08-10 20:03 ` [PATCH v2 1/2] cpumask: introduce " Mike Turquette
2011-08-10 20:53 ` Daniel Lezcano
2011-08-11 6:06 ` Amit Kucheria
2011-08-11 18:39 ` Turquette, Mike
2011-08-10 20:03 ` [PATCH v2 2/2] cpu: update cpu_hotpluggable_mask in register_cpu Mike Turquette
2011-08-10 20:54 ` Daniel Lezcano
2011-08-11 18:30 ` [PATCH v2 0/2] new cpumask for hotpluggable CPUs Peter Zijlstra
2011-08-11 19:25 ` Turquette, Mike
2011-08-11 19:38 ` Peter Zijlstra
2011-08-11 19:54 ` [PATCH v3 1/2] cpumask: introduce " Mike Turquette
2011-08-11 19:54 ` [PATCH v3 2/2] cpu: update cpu_hotpluggable_mask in register_cpu Mike Turquette
2011-08-11 20:02 ` Peter Zijlstra [this message]
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