Messages in this thread | | | Subject | Re: [PATCH] Fix the race between smp_call_function and CPU booting | From | Peter Zijlstra <> | Date | Tue, 20 Mar 2012 14:41:07 +0100 |
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On Tue, 2012-03-20 at 18:44 +0530, Srivatsa S. Bhat wrote: > > > I don't think this patch would change anything, atleast it wouldn't get > rid of the warning that Liu reported. Because, he is running his stress > tests on a machine which has only 2 CPUs. So effectively, we are hotplugging > only CPU1 (since CPU0 can't be taken offline, on Intel boxes). > > Also, CPU1 is removed from cpu_active_mask during CPU_DOWN_PREPARE time itself, > and migrate_tasks() comes much later (during CPU_DYING). And in any case, > dest_cpu will never be CPU1, because it is the CPU that is going down. So it > *has* to be CPU0 anyway. > > So, I don't think changes to select_fallback_rq() to make it more careful is > going to make any difference in the particular scenario that Liu is testing. > > That said, even I don't know what the root cause of the warning is.. :-(
Its a race in cpu-up, we set active before online, when we do a wakeup select_task_rq() will see !cpu_online(), we then call select_fallback_rq() to compute a new cpu, select_fallback_rq() computes a new cpu against cpu_active (which is set) and can thus return cpu 1, even though it is still offline.
So we queue the task on cpu 1 and send a reschedule ipi, at which point we'll get the reported warning.
My change modifies select_fallback_rq() to require online && active.
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