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Date: Mon, 27 Aug 2007 09:25:00 -0700 From: Andrew Morton <akpm@...ux-foundation.org> To: Cliff Wickman <cpw@....com> Cc: ego@...ibm.com, mingo@...e.hu, vatsa@...ibm.com, oleg@...sign.ru, pj@....com, linux-kernel@...r.kernel.org Subject: Re: [PATCH 1/1] V3: hotplug cpu: migrate a task within its cpuset On Mon, 27 Aug 2007 11:07:03 -0500 Cliff Wickman <cpw@....com> wrote: > > Version 3 adds a missing task_rq_lock()/task_rq_unlock() pair. (Oleg found) > > There was discussion about this patch among: > Andrew Morton, Oleg Nesterov, Gautham Shenoy, Rusty Russell > regarding other approaches: > refusing to offline a cpu with tasks pinned to it, or > providing an administrator the ability to assign such tasks to other cpus > > There is indeed an "assumption" in my patch that the cpuset containing a > pinned task's cpu is a better choice than any online cpu. I think that is > a reasonable assumption given the typical construction of a cpuset and the > reason a task is running in a cpuset. > > And there will be coming cases, at least on some architectures, where a > cpu is offlined as a kernel reaction to a hardware error. In that case > would it not be preferrable to re-pin such tasks and let them proceed? > > > > When a cpu is disabled, move_task_off_dead_cpu() is called for tasks > that have been running on that cpu. > > Currently, such a task is migrated: > 1) to any cpu on the same node as the disabled cpu, which is both online > and among that task's cpus_allowed > 2) to any cpu which is both online and among that task's cpus_allowed > > It is typical of a multithreaded application running on a large NUMA system > to have its tasks confined to a cpuset so as to cluster them near the > memory that they share. Furthermore, it is typical to explicitly place such > a task on a specific cpu in that cpuset. And in that case the task's > cpus_allowed includes only a single cpu. > > This patch would insert a preference to migrate such a task to some cpu within > its cpuset (and set its cpus_allowed to its entire cpuset). > > With this patch, migrate the task to: > 1) to any cpu on the same node as the disabled cpu, which is both online > and among that task's cpus_allowed > 2) to any online cpu within the task's cpuset > 3) to any cpu which is both online and among that task's cpus_allowed > <looks at the No more Mr. Nice Guy. code> OK, so we're no worse than we used to be, really. > include/linux/cpuset.h | 5 +++++ > kernel/cpuset.c | 15 ++++++++++++++- > kernel/sched.c | 16 ++++++++++++++++ How do we communicate this new design/feature to our users? Documentation/cpusets.txt, perhaps? Documentation/cpu-hotplug.txt? git-log? ;) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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