Messages in this thread |  | | Date | Mon, 13 Feb 2012 12:39:29 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets |
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On Mon, Feb 13, 2012 at 11:17:53PM +0530, Srivatsa S. Bhat wrote: > On 02/10/2012 10:23 PM, Paul E. McKenney wrote: > > > On Fri, Feb 10, 2012 at 04:52:07PM +0100, Peter Zijlstra wrote: > >> On Thu, 2012-02-09 at 16:11 +0100, Ingo Molnar wrote: > >> > >>>> My understanding of the code is that when a CPU is taken > >>>> offline, it is removed from all the cpusets and then the > >>>> scan_for_empty_cpusets() function is run to move tasks from > >>>> empty cpusets to their parent cpusets. > >>> > >>> Why is that done that way? offlining a CPU should be an > >>> invariant as far as cpusets are concerned. > >> > >> Can't, tasks need to run someplace. There's two choices, add a still > >> online cpu to the now empty cpuset or move the tasks to a parent that > >> still has online cpus. > >> > >> Both are destructive. > > > > OK, I will ask the stupid question... Hey, somebody has to! ;-) > > > > Would it make sense for offlining the last CPU in a cpuset to be > > destructive, but to allow offlining of a non-last CPU to be reversible? > > > > For example, assume that cpuset A has CPUs 0 and 1, and cpuset B has > > 1, 2, and 3. Then offlining any single CPU and then onlining it would > > restore the cpusets to their original state. Offlining both CPUs 0 and 1 > > would be destructive to cpuset A, so that onlining those two CPUs would > > leave any tasks in cpuset A in some ancestor of cpuset A, and would > > leave cpuset A with no assigned CPUs. However, that same operation > > (offlining both CPUs 0 and 1, then onlining them) would restore cpuset > > B to its original state, covering CPUs 1, 2, and 3. > > But how would this scheme help us? During suspend, all non-boot CPUs are > taken offline. Which means, it would be destructive to any cpuset that > didn't originally contain CPU0 (even when using the above scheme). So, upon > resume, it is still not the same as how it was before suspend.
Yep, it would only help for incremental cases. Or if all cpusets had CPU 0 in them. So preserving cpusets across suspend will require a bigger hammer.
Thanx, Paul
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