Messages in this thread | | | Date | Sat, 25 Aug 2007 13:47:40 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/1] hotplug cpu: migrate a task within its cpuset |
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On 08/24, Andrew Morton wrote: > > On Fri, 24 Aug 2007 17:18:06 -0500 > Cliff Wickman <cpw@sgi.com> wrote: > > > 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. > > operator error.. > > > 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 > > Wouldn't it be saner to refuse the offlining request if the CPU has tasks > which cannot be migrated to any other CPU? I mean, the operator has gone > and asked the machine to perform two inconsistent/incompatible things at > the same time.
I don't think so (regardless of this patch and CONFIG_CPUSETS). Any user can bind its process to (say) CPU 4. This shouldn't block cpu-unplug.
Now, let's suppose that this process is a member of some cpuset which contains CPUs 3 and 4, and CPU 4 goes down.
Before this patch, process leaves its ->cpuset and migrates to some "random" any_online_cpu(). With this patch it stays within ->cpuset and migrates to CPU 3.
> Look at it this way. If we were to merge this patch then it would be > logical to also merge a patch which has the following description: > > "if an process attempts to pin itself onto an presently-offlined CPU, > the kernel will choose a different CPU according to <heuristics> and > will pin the process to that CPU instead".
set_cpus_allowed() just returns -EINVAL in that case, this looks a bit more logical.
Oleg.
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