Re: Possible reproduction of CSD locking issue

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Jan 25, 2022 at 12:27:37PM -0600, Corey Minyard wrote:
> We have a customer that had been seeing CSD lock issues on a Centos 7
> kernel (unfortunately).  I couldn't find anything or any kernel changes
> that might fix it, so I was consdering it was the CSD locking issue you
> have been chasing for a while.
> 
> So I backported the debug patches.  And of course, they stopped seeing
> the issue, at least as much, and they had trouble with the extra CPU
> time the debug code took.  But they just reproduced it.  Here are the
> logs:
> 
> Jan 23 23:39:43 worker0 kernel: [285737.522743] csd: Detected non-responsive CSD lock (#1) on CPU#3, waiting 5000000042 ns for CPU#55 flush_tlb_func+0x0/0xb0(0xffff8e0b3e2afbe8).
> Jan 23 23:39:43 worker0 kernel: [285737.522744]  csd: CSD lock (#1) unresponsive.
> Jan 23 23:39:43 worker0 kernel: [285737.522747]  csd: cnt(0000000): 0000->0000 queue
> Jan 23 23:39:43 worker0 kernel: [285737.522748]  csd: cnt(0000001): ffff->0037 idle
> Jan 23 23:39:43 worker0 kernel: [285737.522749]  csd: cnt(63d8dd8): 0003->0037 ipi
> Jan 23 23:39:43 worker0 kernel: [285737.522750]  csd: cnt(63d8dd9): 0003->0037 ping
> Jan 23 23:39:43 worker0 kernel: [285737.522750]  csd: cnt(63d8dda): 0003->ffff pinged
> Jan 23 23:39:43 worker0 kernel: [285737.522751]  csd: cnt(63d8dea): 0035->0037 pinged
> Jan 23 23:39:43 worker0 kernel: [285737.522752]  csd: cnt(63d8deb): ffff->0037 gotipi
> Jan 23 23:39:43 worker0 kernel: [285737.522752]  csd: cnt(63d8dec): ffff->0037 handle
> Jan 23 23:39:43 worker0 kernel: [285737.522753]  csd: cnt(63d8ded): ffff->0037 dequeue (src CPU 0 == empty)
> Jan 23 23:39:43 worker0 kernel: [285737.522754]  csd: cnt(63d8dee): ffff->0037 hdlend (src CPU 0 == early)
> Jan 23 23:39:43 worker0 kernel: [285737.522754]  csd: cnt(63d8e1f): 0003->0037 queue
> Jan 23 23:39:43 worker0 kernel: [285737.522755]  csd: cnt(63d8e20): 0003->0037 ipi
> Jan 23 23:39:43 worker0 kernel: [285737.522756]  csd: cnt(63d8e21): 0003->0037 ping
> Jan 23 23:39:43 worker0 kernel: [285737.522756]  csd: cnt(63d8e22): 0003->0037 queue
> Jan 23 23:39:43 worker0 kernel: [285737.522757]  csd: cnt(63d8e23): 0003->0037 noipi
> Jan 23 23:39:43 worker0 kernel: [285737.522757]  csd: cnt now: 63fe4cd
> Jan 23 23:39:43 worker0 kernel: [285737.522758] Task dump for CPU 55:
> Jan 23 23:39:43 worker0 kernel: [285737.522761] kubelet         R  running task        0 277695      1 0x00080000
> Jan 23 23:39:43 worker0 kernel: [285737.522761] Call Trace:
> Jan 23 23:39:43 worker0 kernel: [285737.522769]  [<ffffffff84376b6a>] ? __schedule+0x46a/0x990
> Jan 23 23:39:43 worker0 kernel: [285737.522774]  [<ffffffff83db6353>] ? context_tracking_user_enter+0x13/0x20
> Jan 23 23:39:43 worker0 kernel: [285737.522776]  [<ffffffff843775b5>] ? schedule_user+0x45/0x50
> Jan 23 23:39:43 worker0 kernel: [285737.522779]  [<ffffffff8437b518>] ? retint_careful+0x16/0x34

Long-running interrupt handler, maybe?  Or am I misinterpreting this
stack trace?

> Jan 23 23:39:43 worker0 kernel: [285737.522780] csd: Re-sending CSD lock (#1) IPI from CPU#03 to CPU#55
> Jan 23 23:39:43 worker0 kernel: [285737.522788] CPU: 3 PID: 54671 Comm: runc:[2:INIT] Kdump: loaded Tainted: G           OE  ------------ T 3.10.0-1062.12.1.rt56.1042.mvista.test.14.el7.x86_64 #1
> Jan 23 23:39:43 worker0 kernel: [285737.522789] Hardware name: Dell Inc. PowerEdge R740/0YWR7D, BIOS 2.9.4 11/06/2020
> Jan 23 23:39:43 worker0 kernel: [285737.522789] Call Trace:
> Jan 23 23:39:43 worker0 kernel: [285737.522793]  [<ffffffff843718ba>] dump_stack+0x19/0x1b
> Jan 23 23:39:43 worker0 kernel: [285737.522798]  [<ffffffff83d0bcd8>] __csd_lock_wait+0x1a8/0x2a0
> Jan 23 23:39:43 worker0 kernel: [285737.522800]  [<ffffffff83c6d870>] ? leave_mm+0x120/0x120
> Jan 23 23:39:43 worker0 kernel: [285737.522802]  [<ffffffff83d0bfa4>] smp_call_function_single+0xc4/0x1b0
> Jan 23 23:39:43 worker0 kernel: [285737.522804]  [<ffffffff83c6d870>] ? leave_mm+0x120/0x120
> Jan 23 23:39:43 worker0 kernel: [285737.522809]  [<ffffffff83e2684b>] ? page_counter_uncharge+0x3b/0x70
> Jan 23 23:39:43 worker0 kernel: [285737.522811]  [<ffffffff83d0c614>] smp_call_function_many+0x344/0x380
> Jan 23 23:39:43 worker0 kernel: [285737.522813]  [<ffffffff83c6d870>] ? leave_mm+0x120/0x120
> Jan 23 23:39:43 worker0 kernel: [285737.522816]  [<ffffffff83c6da38>] native_flush_tlb_others+0xb8/0xc0
> Jan 23 23:39:43 worker0 kernel: [285737.522818]  [<ffffffff83c6dc25>] flush_tlb_page+0x65/0xf0
> Jan 23 23:39:43 worker0 kernel: [285737.522821]  [<ffffffff83dfdf98>] ptep_clear_flush+0x68/0xa0
> Jan 23 23:39:43 worker0 kernel: [285737.522825]  [<ffffffff83de6806>] wp_page_copy.isra.83+0x3d6/0x650
> Jan 23 23:39:43 worker0 kernel: [285737.522828]  [<ffffffff83de8cb4>] do_wp_page+0xb4/0x710
> Jan 23 23:39:43 worker0 kernel: [285737.522832]  [<ffffffff83decbb4>] handle_mm_fault+0x884/0x1340
> Jan 23 23:39:43 worker0 kernel: [285737.522835]  [<ffffffff83cd7799>] ? update_cfs_shares+0xa9/0xf0
> Jan 23 23:39:43 worker0 kernel: [285737.522839]  [<ffffffff8437efc3>] __do_page_fault+0x213/0x5a0
> Jan 23 23:39:43 worker0 kernel: [285737.522841]  [<ffffffff8437f385>] do_page_fault+0x35/0x90
> Jan 23 23:39:43 worker0 kernel: [285737.522842]  [<ffffffff8437b728>] page_fault+0x28/0x30
> Jan 23 23:39:43 worker0 kernel: [285737.522845] csd: CSD lock (#1) got unstuck on CPU#03, CPU#55 released the lock.
> 
> Hopefully this is the issue you are chasing and not something else.
> I've been studying them to see what they mean, but I thought you might
> be interested to get them asap.

Well, there have been several bugs causing these CSD lock issues, so what
is one more?  ;-)

More seriously, have you tried Frederic's patch?  This fixes the issue
described here:  https://paulmck.livejournal.com/62071.html
If your stack above was due to an interrupt storm rather than a
long-running interrupt, this might well be the fix.

Oh, and Jürgen Groß reportedly found an issue about a year ago that
could potentially be related, but I see that he is already on CC.

And, unfortunately, even more seriously, this CSD-lock diagnostic code
will very likely continue to find problems, just as the infamous RCU
CPU stall warnings and hard/soft lockup warnings do.

							Thanx, Paul

------------------------------------------------------------------------

commit 53e87e3cdc155f20c3417b689df8d2ac88d79576
Author: Frederic Weisbecker <frederic@kernel.org>
Date:   Tue Oct 26 16:10:54 2021 +0200

    timers/nohz: Last resort update jiffies on nohz_full IRQ entry
    
    When at least one CPU runs in nohz_full mode, a dedicated timekeeper CPU
    is guaranteed to stay online and to never stop its tick.
    
    Meanwhile on some rare case, the dedicated timekeeper may be running
    with interrupts disabled for a while, such as in stop_machine.
    
    If jiffies stop being updated, a nohz_full CPU may end up endlessly
    programming the next tick in the past, taking the last jiffies update
    monotonic timestamp as a stale base, resulting in an tick storm.
    
    Here is a scenario where it matters:
    
    0) CPU 0 is the timekeeper and CPU 1 a nohz_full CPU.
    
    1) A stop machine callback is queued to execute somewhere.
    
    2) CPU 0 reaches MULTI_STOP_DISABLE_IRQ while CPU 1 is still in
       MULTI_STOP_PREPARE. Hence CPU 0 can't do its timekeeping duty. CPU 1
       can still take IRQs.
    
    3) CPU 1 receives an IRQ which queues a timer callback one jiffy forward.
    
    4) On IRQ exit, CPU 1 schedules the tick one jiffy forward, taking
       last_jiffies_update as a base. But last_jiffies_update hasn't been
       updated for 2 jiffies since the timekeeper has interrupts disabled.
    
    5) clockevents_program_event(), which relies on ktime_get(), observes
       that the expiration is in the past and therefore programs the min
       delta event on the clock.
    
    6) The tick fires immediately, goto 3)
    
    7) Tick storm, the nohz_full CPU is drown and takes ages to reach
       MULTI_STOP_DISABLE_IRQ, which is the only way out of this situation.
    
    Solve this with unconditionally updating jiffies if the value is stale
    on nohz_full IRQ entry. IRQs and other disturbances are expected to be
    rare enough on nohz_full for the unconditional call to ktime_get() to
    actually matter.
    
    Reported-by: Paul E. McKenney <paulmck@kernel.org>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Tested-by: Paul E. McKenney <paulmck@kernel.org>
    Link: https://lore.kernel.org/r/20211026141055.57358-2-frederic@kernel.org

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 322b65d456767..41f470929e991 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -595,7 +595,8 @@ void irq_enter_rcu(void)
 {
 	__irq_enter_raw();
 
-	if (is_idle_task(current) && (irq_count() == HARDIRQ_OFFSET))
+	if (tick_nohz_full_cpu(smp_processor_id()) ||
+	    (is_idle_task(current) && (irq_count() == HARDIRQ_OFFSET)))
 		tick_irq_enter();
 
 	account_hardirq_enter(current);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6bffe5af8cb11..17a283ce2b20f 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1375,6 +1375,13 @@ static inline void tick_nohz_irq_enter(void)
 	now = ktime_get();
 	if (ts->idle_active)
 		tick_nohz_stop_idle(ts, now);
+	/*
+	 * If all CPUs are idle. We may need to update a stale jiffies value.
+	 * Note nohz_full is a special case: a timekeeper is guaranteed to stay
+	 * alive but it might be busy looping with interrupts disabled in some
+	 * rare case (typically stop machine). So we must make sure we have a
+	 * last resort.
+	 */
 	if (ts->tick_stopped)
 		tick_nohz_update_jiffies(now);
 }