On Mon 24-09-12 21:31:35, Borislav Petkov wrote: > On Tue, Sep 25, 2012 at 12:18:46AM +0530, Srivatsa S. Bhat wrote: > > >> Sure thing. > > >> Out of ~25 runs I only triggered it once, without the patch the > > >> trigger-rate is higher. > > >> > > >> [ 55.098249] Broke affinity for irq 81 > > >> [ 55.105108] smpboot: CPU 1 is now offline > > >> [ 55.311216] smpboot: Booting Node 0 Processor 1 APIC 0x11 > > >> [ 55.333022] LVT offset 0 assigned for vector 0x400 > > >> [ 55.545877] smpboot: CPU 2 is now offline > > >> [ 55.753050] smpboot: Booting Node 0 Processor 2 APIC 0x12 > > >> [ 55.775582] LVT offset 0 assigned for vector 0x400 > > >> [ 55.986747] smpboot: CPU 3 is now offline > > >> [ 56.193839] smpboot: Booting Node 0 Processor 3 APIC 0x13 > > >> [ 56.212643] LVT offset 0 assigned for vector 0x400 > > >> [ 56.423201] Got negative events: -25 > > > > > > I see it: > > > > > > __percpu_counter_sum does for_each_online_cpu without doing > > > get/put_online_cpus(). > > > > > > > Maybe I'm missing something, but that doesn't immediately tell me > > what's the exact source of the bug.. Note that there is a hotplug > > callback percpu_counter_hotcpu_callback() that takes the same > > fbc->lock before updating/resetting the percpu counters of offline > > CPU. So, though the synchronization is a bit weird, I don't > > immediately see a problematic race condition there. > > Well, those oopses both happen when a cpu comes online. > > According to when percpu_counter_hotcpu_callback is run (at CPU_DEAD) > then those percpu variables should have correctly updated values. > > So there has to be some other case where we read garbage which is a > negative value - otherwise we wouldn't be seeing the debug output. > > For example, look at the log output above: we bring down cpu 3 just to > bring it right back online. So there has to be something fishy along > that codepath... Well, I think the race happens when a CPU is dying and we call percpu_counter_sum() after it is marked offline but before callbacks are run. percpu_counter_sum() then does not add died CPU's counter in the sum and thus total can go negative. If get/put_online_cpus() fixes this race, I'd be happy. OTOH in theory, percpu_counter_sum() can return negative values even without CPU hotplug when percpu_counter_sum() races with cpu local operations. It cannot happen with the current flexible proportion code but I think making the code more robust is a good idea. I'll send a patch for this. Still fixing the percpu counters would be nice as these races could cause random errors to computed proportions and that's bad for writeback. Honza -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>