linux-kernel - Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.

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Date:	Mon, 22 Jun 2009 01:08:51 -0500
From:	Nathan Lynch <ntl@...ox.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	svaidy@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	linux-kernel@...r.kernel.org, Balbir Singh <balbir@...ibm.com>,
	Rusty Russel <rusty@...tcorp.com.au>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Shoahua Li <shaohua.li@...ux.com>
Subject: Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.

Ingo Molnar <mingo@...e.hu> writes:

> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
>> On Wed, 2009-06-17 at 17:07 +0200, Ingo Molnar wrote:
>> > * Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>> > 
>> > > On Wed, Jun 17, 2009 at 09:32:57AM +0200, Peter Zijlstra wrote:
>> > > > On Tue, 2009-06-16 at 13:37 +0530, Vaidyanathan Srinivasan wrote:
>> > > > > * Andrew Morton <akpm@...ux-foundation.org> [2009-06-15 23:23:18]:
>> > > > > 
>> > > > > > On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy <ego@...ibm.com> wrote:
>> > > > > > 
>> > > > > > > Currently on a ppc64 box with 16 CPUs, the time taken for
>> > > > > > > a individual cpu-hotplug operation is as follows.
>> > > > > > > 
>> > > > > > >     # time echo 0 > /sys/devices/system/cpu/cpu2/online
>> > > > > > >     real    0m0.025s
>> > > > > > >     user    0m0.000s
>> > > > > > >     sys     0m0.002s
>> > > > > > > 
>> > > > > > >     # time echo 1 > /sys/devices/system/cpu/cpu2/online
>> > > > > > >     real    0m0.021s
>> > > > > > >     user    0m0.000s
>> > > > > > >     sys     0m0.000s
>> > > > > > 
>> > > > > > Surprised.  Do people really online and offline CPUs frequently enough
>> > > > > > for this to be a problem?
>> > > > > 
>> > > > > Certainly not for hardware faults or hardware replacement, but
>> > > > > cpu-hotplug interface is useful for changing system configuration to
>> > > > > meet different objectives like
>> > > > > 
>> > > > > * Reduce system capacity to reduce average power and reduce heat
>> > > > > 
>> > > > > * Increasing number of cores and threads in a CPU package is leading
>> > > > >   to multiple cpu offline/online operations for any perceivable effect
>> > > > > 
>> > > > > * Dynamically change CPU configurations in virtualized environments
>> > > > 
>> > > > I tend to agree with Andrew, if any of those things are done 
>> > > > frequent enough that the hotplug performance matter you're doing 
>> > > > something mighty odd.
>> > > 
>> > > Boot speedup?
>> > 
>> > Also, if it brings more attention (and more stability and more 
>> > bugfixes) to CPU hotplug that's only good.
>> 
>> Sure, but do we need the extra complexity?
>> 
>> I mean, sure bootup speed might be nice, but any of the scenarios 
>> given should simply not require cpu hotplug actions of a frequent 
>> enough nature that any performance matters.
>
> Well, the fact that the patches exist show that there's people 
> caring about the speedup here. The speedup itself is non-trivial.

If I correctly understand the behavior of the patch set as posted, there
is no speedup beyond eliminating the overhead of multiple writes to
/sys/devices/system/cpu/cpu*/online.  To obtain non-trivial speedups via
bulk hotplug, one or both of the following items from the TODO list need
to be completed:

- Enhance the subsystem notifiers to work on a cpumask_var_t instead of a cpu
  id.

- Optimize the subsystem notifiers to reduce the time consumed while
  handling CPU_[DOWN_PREPARE/DEAD/UP_PREPARE/ONLINE] events for the
  cpumask_var_t.

Right?

(The powerpc-specific patch at the beginning of the series improves
hot-online time for a single cpu in some circumstances and is basically
unrelated to the aim of the patch set -- it should go upstream through
the powerpc tree independently.)
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