On Fri, Dec 19, 2014 at 11:15:21AM -0800, Linus Torvalds wrote: > Here's another pattern. In your latest thing, every single time that > CPU1 is waiting for some other CPU to pick up the IPI, we have CPU0 > doing this: > > [24998.060963] NMI backtrace for cpu 0 > [24998.061989] CPU: 0 PID: 2940 Comm: trinity-c150 Not tainted 3.18.0+ #108 > [24998.064073] task: ffff8801bf3536b0 ti: ffff880197e0c000 task.ti: > ffff880197e0c000 > [24998.065137] RIP: 0010:[<ffffffff8103e006>] [<ffffffff8103e006>] > read_hpet+0x16/0x20 > [24998.083577] [<ffffffff810e0d3e>] ktime_get+0x3e/0xa0 > [24998.084450] [<ffffffff810e9cd3>] tick_sched_timer+0x23/0x160 > [24998.085315] [<ffffffff810daf96>] __run_hrtimer+0x76/0x1f0 > [24998.086173] [<ffffffff810e9cb0>] ? tick_init_highres+0x20/0x20 > [24998.087025] [<ffffffff810db2e7>] hrtimer_interrupt+0x107/0x260 > [24998.087877] [<ffffffff81031a4b>] local_apic_timer_interrupt+0x3b/0x70 > [24998.088732] [<ffffffff8179bca5>] smp_apic_timer_interrupt+0x45/0x60 > [24998.089583] [<ffffffff8179a0df>] apic_timer_interrupt+0x6f/0x80 > [24998.090435] <EOI> > [24998.091279] [<ffffffff810da66e>] ? __remove_hrtimer+0x4e/0xa0 > [24998.092118] [<ffffffff812c7c7a>] ? ipcget+0x8a/0x1e0 > [24998.092951] [<ffffffff812c7c6c>] ? ipcget+0x7c/0x1e0 > [24998.093779] [<ffffffff812c8d6d>] SyS_msgget+0x4d/0x70 > > and I think that's the smoking gun. The reason CPU0 isn't picking up > any IPI's is because it is in some endless loop around read_hpet(). > > There is even time information in the register dump: > > RAX: 0000000061fece8a RBX: 0000000000510792 RCX: 0000000000000000 > RAX: 0000000079e588fc RBX: 0000000000511d6e RCX: 0000000000000000 > RAX: 0000000091ca7f65 RBX: 0000000000513346 RCX: 0000000000000000 > RAX: 00000000a9afbd0d RBX: 000000000051491e RCX: 0000000000000000 > RAX: 00000000cbd1340c RBX: 000000000051684a RCX: 0000000000000000 > RAX: 00000000fb9d303f RBX: 00000000005193fc RCX: 0000000000000000 > RAX: 000000002b67efe4 RBX: 000000000051c224 RCX: 0000000000000004 > > That RAX value is the value we just read from the HPET, and RBX seems > to be monotonically increasing too, so it's likely the sequence > counter in ktime_get(). > > So it's not stuck *inside* read_hpet(), and it's almost certainly not > the loop over the sequence counter in ktime_get() either (it's not > increasing *that* quickly). But some basically infinite __run_hrtimer > thing or something?
Really interesting. So, we're calling __run_hrtimer in a loop: while ((node = timerqueue_getnext(&base->active))) { ... __run_hrtimer(timer, &basenow); ... } The easy question is how often does trinity call nanosleep? Looking at __run_hrtimer(), it drops the lock and runs the function and then takes the lock again, maybe enqueueing us again right away. timer->state is supposed to protect us from other CPUs jumping in and doing something else with the timer, but it feels racey wrt remove_hrtimer(). Something like this, but I'm not sure how often __hrtimer_start_range_ns gets called CPU 0 CPU 1 __run_hrtimer() timer->state = HRTIMER_STATE_CALLBACK removed from list unlock cpu_base->lock restrt = fn(timer) __hrtimer_start_range_ns() base = lock_hrtimer_base() ret = remove_hrtimer() finds timer->state = HRTIMER_STATE_CALLBACK does nothing new_base = switch_hrtimer_base() now we're on a different base, different lock lock(cpu_base->lock) enqueue the timer enqueue the timer -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/