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McKenney" To: Marco Elver Cc: Steven Rostedt , Anders Roxell , Andrew Morton , Alexander Potapenko , Dmitry Vyukov , Jann Horn , Mark Rutland , Linux Kernel Mailing List , Linux-MM , kasan-dev , rcu@vger.kernel.org, Peter Zijlstra , Tejun Heo , Lai Jiangshan Subject: Re: [PATCH] kfence: Avoid stalling work queue task without allocations Message-ID: <20201119184854.GY1437@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20201112161439.GA2989297@elver.google.com> <20201112175406.GF3249@paulmck-ThinkPad-P72> <20201113175754.GA6273@paulmck-ThinkPad-P72> <20201117105236.GA1964407@elver.google.com> <20201117182915.GM1437@paulmck-ThinkPad-P72> <20201118225621.GA1770130@elver.google.com> <20201118233841.GS1437@paulmck-ThinkPad-P72> <20201119125357.GA2084963@elver.google.com> <20201119151409.GU1437@paulmck-ThinkPad-P72> <20201119170259.GA2134472@elver.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201119170259.GA2134472@elver.google.com> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Nov 19, 2020 at 06:02:59PM +0100, Marco Elver wrote: > On Thu, Nov 19, 2020 at 07:14AM -0800, Paul E. McKenney wrote: > > On Thu, Nov 19, 2020 at 01:53:57PM +0100, Marco Elver wrote: > > > On Wed, Nov 18, 2020 at 03:38PM -0800, Paul E. McKenney wrote: > > > > On Wed, Nov 18, 2020 at 11:56:21PM +0100, Marco Elver wrote: > > > > > [...] > > > > > I think I figured out one piece of the puzzle. Bisection keeps pointing > > > > > me at some -rcu merge commit, which kept throwing me off. Nor did it > > > > > help that reproduction is a bit flaky. However, I think there are 2 > > > > > independent problems, but the manifestation of 1 problem triggers the > > > > > 2nd problem: > > > > > > > > > > 1. problem: slowed forward progress (workqueue lockup / RCU stall reports) > > > > > > > > > > 2. problem: DEADLOCK which causes complete system lockup > > > > > > > > > > | ... > > > > > | CPU0 > > > > > | ---- > > > > > | lock(rcu_node_0); > > > > > | > > > > > | lock(rcu_node_0); > > > > > | > > > > > | *** DEADLOCK *** > > > > > | > > > > > | 1 lock held by event_benchmark/105: > > > > > | #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: print_other_cpu_stall kernel/rcu/tree_stall.h:493 [inline] > > > > > | #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: check_cpu_stall kernel/rcu/tree_stall.h:652 [inline] > > > > > | #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_pending kernel/rcu/tree.c:3752 [inline] > > > > > | #0: ffffbb6e0b804458 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x428/0xd40 kernel/rcu/tree.c:2581 > > > > > | ... > > > > > > > > > > Problem 2 can with reasonable confidence (5 trials) be fixed by reverting: > > > > > > > > > > rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled > > > > > > > > > > At which point the system always boots to user space -- albeit with a > > > > > bunch of warnings still (attached). The supposed "good" version doesn't > > > > > end up with all those warnings deterministically, so I couldn't say if > > > > > the warnings are expected due to recent changes or not (Arm64 QEMU > > > > > emulation, 1 CPU, and lots of debugging tools on). > > > > > > > > > > Does any of that make sense? > > > > > > > > Marco, it makes all too much sense! :-/ > > > > > > > > Does the patch below help? > > > > > > > > Thanx, Paul > > > > > > > > ------------------------------------------------------------------------ > > > > > > > > commit 444ef3bbd0f243b912fdfd51f326704f8ee872bf > > > > Author: Peter Zijlstra > > > > Date: Sat Aug 29 10:22:24 2020 -0700 > > > > > > > > sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled > > > > > > My assumption is that this is a replacement for "rcu: Don't invoke > > > try_invoke_on_locked_down_task() with irqs disabled", right? > > > > Hmmm... It was actually intended to be in addition. > > > > > That seems to have the same result (same test setup) as only reverting > > > "rcu: Don't invoke..." does: still results in a bunch of workqueue > > > lockup warnings and RCU stall warnings, but boots to user space. I > > > attached a log. If the warnings are expected (are they?), then it looks > > > fine to me. > > > > No, they are not at all expected, but might be a different symptom > > of the original problem. Please see below. > > > > > (And just in case: with "rcu: Don't invoke..." and "sched/core: > > > Allow..." both applied I still get DEADLOCKs -- but that's probably > > > expected.) > > > > As noted earlier, it is a surprise. Could you please send me the > > console output? > > I've attached the output of a run with both commits applied. Got it, thank you! > > > Testing all events: OK > > > hrtimer: interrupt took 17120368 ns > > > Running tests again, along with the function tracer > > > Running tests on all trace events: > > > Testing all events: > > > BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 12s! > > > Showing busy workqueues and worker pools: > > > workqueue events: flags=0x0 > > > pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2 > > > pending: vmstat_shepherd > > > BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 17s! > > > Showing busy workqueues and worker pools: > > > workqueue events: flags=0x0 > > > pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2 > > > pending: vmstat_shepherd > > > workqueue events_power_efficient: flags=0x82 > > > pwq 2: cpus=0 flags=0x4 nice=0 active=1/256 refcnt=3 > > > pending: neigh_periodic_work > > > ------------[ cut here ]------------ > > > WARNING: CPU: 0 PID: 1 at kernel/rcu/tree_stall.h:758 rcu_check_gp_start_stall kernel/rcu/tree_stall.h:750 [inline] > > > WARNING: CPU: 0 PID: 1 at kernel/rcu/tree_stall.h:758 rcu_check_gp_start_stall.isra.0+0x14c/0x210 kernel/rcu/tree_stall.h:711 > > > > I have different line numbering, > > This is still using next-20201110. I'll rerun with latest -next as well. No problem, as it looks like next-20201105 is a reasonable approximation. > > but the only warning that I see in this > > function is the one complaining that RCU has been ignoring a request to > > start a grace period for too long. This usually happens because the RCU > > grace-period kthread (named "rcu_preempt" in your case, but can also be > > named "rcu_sched") is being prevented from running, but can be caused > > by other things as well. > > > > > Modules linked in: > > > CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.10.0-rc3-next-20201110-00003-g920304642405-dirty #30 > > > Hardware name: linux,dummy-virt (DT) > > > pstate: 20000085 (nzCv daIf -PAN -UAO -TCO BTYPE=--) > > > pc : rcu_check_gp_start_stall kernel/rcu/tree_stall.h:750 [inline] > > > pc : rcu_check_gp_start_stall.isra.0+0x14c/0x210 kernel/rcu/tree_stall.h:711 > > > lr : __xchg_mb arch/arm64/include/asm/cmpxchg.h:88 [inline] > > > lr : atomic_xchg include/asm-generic/atomic-instrumented.h:615 [inline] > > > lr : rcu_check_gp_start_stall kernel/rcu/tree_stall.h:751 [inline] > > > lr : rcu_check_gp_start_stall.isra.0+0x148/0x210 kernel/rcu/tree_stall.h:711 > > > > Two program counters and four link registers? Awesome! ;-) > > Ah I'm using syzkaller's symbolizer, which duplicates lines if there was > an inline function (remove all the "[inline]" and it should make sense, > but the "[inline]" tell you the actual line). Obviously for things like > this it's a bit unintuitive. :-) Very useful, though, and a big THANK YOU to those who made it happen! > > > sp : ffff800010003d20 > > > x29: ffff800010003d20 x28: ffff274ac3a10000 > > > x27: 0000000000000000 x26: ffff274b3dbe72d8 > > > x25: ffffbcb867722000 x24: 0000000000000000 > > > x23: 0000000000000000 x22: ffffbcb8681d1260 > > > x21: ffffbcb86735b000 x20: ffffbcb867404440 > > > x19: ffffbcb867404440 x18: 0000000000000123 > > > x17: ffffbcb865d400f0 x16: 0000000000000002 > > > x15: 0000000000000002 x14: 0000000000000000 > > > x13: 003d090000000000 x12: 00001e8480000000 > > > x11: ffffbcb867958980 x10: ffff800010003cf0 > > > x9 : ffffbcb864f4b7c8 x8 : 0000000000000080 > > > x7 : 0000000000000026 x6 : ffffbcb86774e4c0 > > > x5 : 0000000000000000 x4 : 00000000d4001f4b > > > x3 : 0000000000000000 x2 : 0000000000000000 > > > x1 : 0000000000000001 x0 : 0000000000000000 > > > Call trace: > > > rcu_check_gp_start_stall kernel/rcu/tree_stall.h:750 [inline] > > > rcu_check_gp_start_stall.isra.0+0x14c/0x210 kernel/rcu/tree_stall.h:711 > > > rcu_core+0x168/0x9e0 kernel/rcu/tree.c:2719 > > > rcu_core_si+0x18/0x28 kernel/rcu/tree.c:2737 > > > > The RCU_SOFTIRQ handler is causing this checking to occur, for whatever > > that is worth. > > > > > __do_softirq+0x188/0x6b4 kernel/softirq.c:298 > > > do_softirq_own_stack include/linux/interrupt.h:568 [inline] > > > invoke_softirq kernel/softirq.c:393 [inline] > > > __irq_exit_rcu kernel/softirq.c:423 [inline] > > > irq_exit+0x1cc/0x1e0 kernel/softirq.c:447 > > > __handle_domain_irq+0xb4/0x130 kernel/irq/irqdesc.c:690 > > > handle_domain_irq include/linux/irqdesc.h:170 [inline] > > > gic_handle_irq+0x70/0x108 drivers/irqchip/irq-gic.c:370 > > > el1_irq+0xc0/0x180 arch/arm64/kernel/entry.S:651 > > > arch_local_irq_restore+0x8/0x10 arch/arm64/include/asm/irqflags.h:124 > > > release_probes kernel/tracepoint.c:113 [inline] > > > tracepoint_remove_func kernel/tracepoint.c:315 [inline] > > > tracepoint_probe_unregister+0x220/0x378 kernel/tracepoint.c:382 > > > trace_event_reg+0x58/0x150 kernel/trace/trace_events.c:298 > > > __ftrace_event_enable_disable+0x424/0x608 kernel/trace/trace_events.c:412 > > > ftrace_event_enable_disable kernel/trace/trace_events.c:495 [inline] > > > __ftrace_set_clr_event_nolock+0x120/0x180 kernel/trace/trace_events.c:811 > > > __ftrace_set_clr_event+0x60/0x90 kernel/trace/trace_events.c:833 > > > event_trace_self_tests+0xd4/0x114 kernel/trace/trace_events.c:3661 > > > event_trace_self_test_with_function kernel/trace/trace_events.c:3734 [inline] > > > event_trace_self_tests_init+0x88/0xa8 kernel/trace/trace_events.c:3747 > > > do_one_initcall+0xa4/0x500 init/main.c:1212 > > > do_initcall_level init/main.c:1285 [inline] > > > do_initcalls init/main.c:1301 [inline] > > > do_basic_setup init/main.c:1321 [inline] > > > kernel_init_freeable+0x344/0x3c4 init/main.c:1521 > > > kernel_init+0x20/0x16c init/main.c:1410 > > > ret_from_fork+0x10/0x34 arch/arm64/kernel/entry.S:961 > > > irq event stamp: 3274113 > > > hardirqs last enabled at (3274112): [] rcu_core+0x974/0x9e0 kernel/rcu/tree.c:2716 > > > hardirqs last disabled at (3274113): [] __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:108 [inline] > > > hardirqs last disabled at (3274113): [] _raw_spin_lock_irqsave+0xb8/0x14c kernel/locking/spinlock.c:159 > > > softirqs last enabled at (3272576): [] __do_softirq+0x630/0x6b4 kernel/softirq.c:325 > > > softirqs last disabled at (3274101): [] do_softirq_own_stack include/linux/interrupt.h:568 [inline] > > > softirqs last disabled at (3274101): [] invoke_softirq kernel/softirq.c:393 [inline] > > > softirqs last disabled at (3274101): [] __irq_exit_rcu kernel/softirq.c:423 [inline] > > > softirqs last disabled at (3274101): [] irq_exit+0x1cc/0x1e0 kernel/softirq.c:447 > > > ---[ end trace 902768efebf5a607 ]--- > > > rcu: rcu_preempt: wait state: RCU_GP_WAIT_GPS(1) ->state: 0x0 delta ->gp_activity 4452 ->gp_req_activity 3848 ->gp_wake_time 3848 ->gp_wake_seq 2696 ->gp_seq 2696 ->gp_seq_needed 2700 ->gp_flags 0x1 > > > > The last thing that RCU's grace-period kthread did was to go to sleep > > waiting for a grace-period request (RCU_GP_WAIT_GPS). > > > > > rcu: rcu_node 0:0 ->gp_seq 2696 ->gp_seq_needed 2700 > > > rcu: RCU callbacks invoked since boot: 2583 > > > rcu_tasks: RTGS_WAIT_CBS(11) since 567120 g:1 i:0/0 k. > > > rcu_tasks_rude: RTGS_WAIT_CBS(11) since 567155 g:1 i:0/1 k. > > > rcu_tasks_trace: RTGS_INIT(0) since 4295464549 g:0 i:0/0 k. N0 h:0/0/0 > > > rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: > > > (detected by 0, t=3752 jiffies, g=2705, q=8) > > > rcu: All QSes seen, last rcu_preempt kthread activity 557 (4295471128-4295470571), jiffies_till_next_fqs=1, root ->qsmask 0x0 > > > rcu: rcu_preempt kthread starved for 557 jiffies! g2705 f0x2 RCU_GP_CLEANUP(7) ->state=0x0 ->cpu=0 > > > > And here we see that RCU's grace-period kthread has in fact been starved. > > > > This kthread is now in RCU_GP_CLEANUP, perhaps because of the wakeup that is > > sent in rcu_check_gp_kthread_starvation(). > > > > My current guess is that this is a consequence of the earlier failures, > > but who knows? > > I can try bisection again, or reverting some commits that might be > suspicious? But we'd need some selection of suspicious commits. The report claims that one of the rcu_node ->lock fields is held with interrupts enabled, which would indeed be bad. Except that all of the stack traces that it shows have these locks held within the scheduling-clock interrupt handler. Now with the "rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled" but without the "sched/core: Allow try_invoke_on_locked_down_task() with irqs disabled" commit, I understand why. With both, I don't see how this happens. At this point, I am reduced to adding lockdep_assert_irqs_disabled() calls at various points in that code, as shown in the patch below. At this point, I would guess that your first priority would be the initial bug rather than this following issue, but you never know, this might well help diagnose the initial bug. Thanx, Paul ------------------------------------------------------------------------ commit ccedf00693ef60f7c06d23490fc41bb60dd43dc3 Author: Paul E. McKenney Date: Thu Nov 19 10:13:06 2020 -0800 rcu: Add lockdep_assert_irqs_disabled() to rcu_sched_clock_irq() and callees This commit adds a number of lockdep_assert_irqs_disabled() calls to rcu_sched_clock_irq() and a number of the functions that it calls. The point of this is to help track down a situation where lockdep appears to be insisting that interrupts are enabled within these functions, which should only ever be invoked from the scheduling-clock interrupt handler. Link: https://lore.kernel.org/lkml/20201111133813.GA81547@elver.google.com/ Signed-off-by: Paul E. McKenney diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 2b3274c..1d956f9 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2580,6 +2580,7 @@ static void rcu_do_batch(struct rcu_data *rdp) void rcu_sched_clock_irq(int user) { trace_rcu_utilization(TPS("Start scheduler-tick")); + lockdep_assert_irqs_disabled(); raw_cpu_inc(rcu_data.ticks_this_gp); /* The load-acquire pairs with the store-release setting to true. */ if (smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) { @@ -2593,6 +2594,7 @@ void rcu_sched_clock_irq(int user) rcu_flavor_sched_clock_irq(user); if (rcu_pending(user)) invoke_rcu_core(); + lockdep_assert_irqs_disabled(); trace_rcu_utilization(TPS("End scheduler-tick")); } @@ -3761,6 +3763,8 @@ static int rcu_pending(int user) struct rcu_data *rdp = this_cpu_ptr(&rcu_data); struct rcu_node *rnp = rdp->mynode; + lockdep_assert_irqs_disabled(); + /* Check for CPU stalls, if enabled. */ check_cpu_stall(rdp); diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 642ebd6..f7aa41c 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -682,6 +682,7 @@ static void rcu_flavor_sched_clock_irq(int user) { struct task_struct *t = current; + lockdep_assert_irqs_disabled(); if (user || rcu_is_cpu_rrupt_from_idle()) { rcu_note_voluntary_context_switch(current); } diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h index 4e3aecd..f276d8e 100644 --- a/kernel/rcu/tree_stall.h +++ b/kernel/rcu/tree_stall.h @@ -267,6 +267,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) struct task_struct *t; struct task_struct *ts[8]; + lockdep_assert_irqs_disabled(); if (!rcu_preempt_blocked_readers_cgp(rnp)) return 0; pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):", @@ -291,6 +292,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags) ".q"[rscr.rs.b.need_qs], ".e"[rscr.rs.b.exp_hint], ".l"[rscr.on_blkd_list]); + lockdep_assert_irqs_disabled(); put_task_struct(t); ndetected++; } @@ -527,6 +529,8 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) struct rcu_node *rnp; long totqlen = 0; + lockdep_assert_irqs_disabled(); + /* Kick and suppress, if so configured. */ rcu_stall_kick_kthreads(); if (rcu_stall_is_suppressed()) @@ -548,6 +552,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps) } } ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock. + lockdep_assert_irqs_disabled(); } for_each_possible_cpu(cpu) @@ -594,6 +599,8 @@ static void print_cpu_stall(unsigned long gps) struct rcu_node *rnp = rcu_get_root(); long totqlen = 0; + lockdep_assert_irqs_disabled(); + /* Kick and suppress, if so configured. */ rcu_stall_kick_kthreads(); if (rcu_stall_is_suppressed()) @@ -649,6 +656,7 @@ static void check_cpu_stall(struct rcu_data *rdp) unsigned long js; struct rcu_node *rnp; + lockdep_assert_irqs_disabled(); if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) || !rcu_gp_in_progress()) return;