Messages in this thread | | | Date | Tue, 20 Mar 2012 20:32:44 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [PATCH] Fix the race between smp_call_function and CPU booting |
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On 03/20/2012 07:11 PM, Peter Zijlstra wrote:
> 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. >
Ok, that makes sense.. Thanks a lot for the explanation!
Regards, Srivatsa S. Bhat
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