Messages in this thread | | | Date | Thu, 4 Jun 2015 13:25:49 -0400 | From | Chris Metcalf <> | Subject | Re: [PATCH v10 1/3] smpboot: allow excluding cpus from the smpboot threads |
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On 05/01/2015 05:23 PM, Frederic Weisbecker wrote: > On Fri, May 01, 2015 at 03:57:51PM -0400, Chris Metcalf wrote: >> On 05/01/2015 04:53 AM, Frederic Weisbecker wrote: >>>> + /* Unpark any threads that were voluntarily parked. */ >>>> + for_each_cpu_not(cpu, &ht->cpumask) { >>>> + if (cpu_online(cpu)) { >>>> + struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu); >>>> + if (tsk) >>>> + kthread_unpark(tsk); >>> I'm still not clear why we are doing that. kthread_stop() should be able >>> to handle parked kthreads, otherwise it needs to be fixed. >> Checking without the unpark, it's actually only a problem with nohz_full. >> In a system without nohz_full, the kthreads are able to stop even when >> they are parked; it's only in the nohz_full case that things wedge. >> For example, booting with only cpu 0 as a housekeeping core (and >> therefore all watchdogs 1-35 on my 36-core tilegx are parked), and >> immediately doing "echo 0 > /proc/sys/kernel/watchdog", I see >> (via SysRq ^O-l) the first parked watchdog, on cpu 1, hung with: >> >> frame 0: 0xfffffff7000f2928 lock_hrtimer_base+0xb8/0xc0 >> frame 1: 0xfffffff7000f2a28 hrtimer_try_to_cancel+0x40/0x170 >> frame 2: 0xfffffff7000f2a28 hrtimer_try_to_cancel+0x40/0x170 >> frame 3: 0xfffffff7000f2b98 hrtimer_cancel+0x40/0x68 >> frame 4: 0xfffffff70014cce0 watchdog_disable+0x50/0x70 >> frame 5: 0xfffffff70008c2d0 smpboot_thread_fn+0x350/0x438 >> frame 6: 0xfffffff700084b28 kthread+0x160/0x178
I finally had some time to look into this issue some more.
With PROVE_LOCKING enabled (after a fix I'll send to LKML shortly), we get no warnings, and ^O-d to print locks shows:
Showing all locks held in the system: 3 locks held by watchdog/1/15: #0: (&(&hp->lock)->rlock){-.....}, at: [<fffffff700620740>] hvc_poll+0xb8/0x4b8 #1: (rcu_read_lock){......}, at: [<fffffff70061d710>] __handle_sysrq+0x0/0x440 #2: (tasklist_lock){.+.+..}, at: [<fffffff7000d7310>] debug_show_all_locks+0xc0/0x350 3 locks held by sh/1732: #0: (sb_writers#4){.+.+.+}, at: [<fffffff70022f6b8>] vfs_write+0x268/0x2c0 #1: (watchdog_proc_mutex){+.+.+.}, at: [<fffffff70016f368>] proc_watchdog_common+0x78/0x1c8 #2: (smpboot_threads_lock){+.+.+.}, at: [<fffffff700093558>] smpboot_unregister_percpu_thread+0x48/0x88
All the watchdog/1/15 locks are attributable to the fact that it's running on the same core that ended up handling the "^O-d" request from SysRq.
The sh process from which I ran the echo eventually shows up as "blocked for more than 120 seconds" and pretty much where you'd expect it to be, waiting on a completion in kthread_stop() at kthread.c:473.
I instrumented lock_hrtimer_base(), and timer->base is null, and never gets set non-null, so the loop spins forever. Perhaps something in nohz is preventing the timer->base from being set?
I'm happy to keep debugging this but I'm not really clear on what could be going wrong here. Any ideas?
-- Chris Metcalf, EZChip Semiconductor http://www.ezchip.com
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