Messages in this thread | | | Date | Mon, 2 Feb 2004 16:40:40 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 3/4] 2.6.2-rc2-mm2 CPU Hotplug: The Core |
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* Srivatsa Vaddagiri <vatsa@in.ibm.com> wrote:
> One possibility is the migration request will be in CPU 0's queue and > not in CPU1's? Hence there won't be anything (in CPU1's migration > request queue) to process when CPU1 is brought down. However by the > time migration thread on CPU 0 gets around to processing the request, > CPU1 is already down and hence the check.
IMO the semantics of this area are not defined clearly enough, hence these problems. __migrate_task(cpu) is a wrapper around set_cpus_allowed(). If a CPU goes away atomically then the only source of some non-online CPU is from user-space or unsafe references of smp_processor_id() [which are also preempt-unsafe so need to be fixed anyway]. set_cpus_allowed() deals with invalid masks just fine: it does nothing and returns an error. The following sequence shouldnt be possible to trigger if CPU-down is an atomic event:
/* Affinity changed (again). */ if (!cpu_isset(dest_cpu, p->cpus_allowed)) goto out; /* CPU went down. */ if (cpu_is_offline(dest_cpu)) goto out;
because p->cpus_allowed is fixed up atomically. If this task is in CPU1's migration queue then the CPU-down mechanism takes care of it. If this task is in CPU0's migration queue then the task will be migrated off CPU1 by the CPU-down mechanism and CPU0's migration wont have alot of work left.
kernel-space threads that rely on running on a specific CPU need the callback mechanism on CPU-down, so there's no problem with them.
user-space tasks that rely on running on a specific CPU need a callback too. (probably in the form of a signal, which, if unhandled, terminates the task.) Eg. if a webserver has a mode to run one thread per CPU, then the server needs to adapt to the new situation when a CPU goes away. We cannot just unilaterally migrate a task and violate its affinity.
another thing: if the migrate-irqs op is done atomically too (together with the migrate-tasks op) then the special-cases in idle_balance() and rebalance_tick() could go away too.
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