Decoding Those Inscrutable RCU CPU Stall Warnings
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- Charla Barbra Newman
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1 Paul E. McKenney, IBM Distinguished Engineer, Linux Technology Center Member, IBM Academy of Technology Open Source Summit North America, September 12, 2017 Decoding Those Inscrutable RCU CPU Stall Warnings They are for your own good! Honest!!!
2 Overview Why does RCU emit CPU stall warnings? How to decode CPU stall warnings What causes CPU stalls? Obligatory war story Summary 2
3 Why Does RCU Emit CPU Stall Warnings? 3
4 RCU Mostly Doesn't Emit CPU Stall Warnings!!! CPU 0 RCU Reader Idle CPU 1 RCU Reader Userspace Execution RCU Grace period done Time 4
5 When Does RCU Emit CPU Stall Warnings? When RCU Believes That a CPU is in Deep Trouble!!! At Least 21 Seconds CPU 0 RCU Reader (Including Interrupts Disabled) CPU 1 RCU Reader Userspace Execution RCU RCU CPU Stall Warning Time 5
6 But What if I Don't Want CPU Stall Warnings??? 6
7 But What if I Don't Want CPU Stall Warnings??? Boot with rcupdate.rcu_cpu_stall_suppress=1 Boot with rcupdate.rcu_cpu_stall_timeout=nn (in seconds) Or build with CONFIG_RCU_CPU_STALL_TIMEOUT=NN 3 <= N <= 300, in seconds Why would you want to suppress CPU stall warnings? Slow embedded system tested on faster development system Increase timeout on slow embedded system (or decrease during test) Throughput-based rip-and-replace cloud-computing environment Embedded production environment where console output is ignored Suppress warnings entirely But if response time matters, you care about CPU stalls Especially during development and testing 7
8 How to Decode CPU Stall Warnings 8
9 Example RCU CPU Stall Warning Splat (First Format) INFO: rcu_sched detected stalls on CPUs/tasks: : (1 GPs behind) idle=bf2/ /0 softirq=554/555 fqs=6754 (detected by 1, t=21003 jiffies, g=-154, c=-155, q=120339) Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 773 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 task: ffff93f7ddd172c0 task.stack: ffff95a3417f4000 RIP: 0010:get_seconds+0xc/0x10 RSP: 0000:ffff95a3417f7ef0 EFLAGS: RAX: a853e2 RBX: a853e6 RCX: ffffffff8bc45d98 RDX: RSI: RDI: ffffffff8cf7f34c RBP: ffff95a3417f7f00 R08: fffffffe R09: e R10: R11: a R12: ffff93f7ddfae100 R13: ffff95a3400d7cf0 R14: R15: ffff93f7ddd172c0 FS: (0000) GS:ffff93f7dfc00000(0000) knlgs: CS: 0010 DS: 0000 ES: 0000 CR0: CR2: CR3: a0a000 CR4: f0 Call Trace:? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 9
10 Identifying an RCU CPU Stall Warning (First Format) INFO: rcu_sched detected stalls on CPUs/tasks: : (1 GPs behind) idle=bf2/ /0 softirq=554/555 fqs=6754 (detected by 1, t=21003 jiffies, g=-154, c=-155, q=120339) Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 773 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 task: ffff93f7ddd172c0 task.stack: ffff95a3417f4000 RIP: 0010:get_seconds+0xc/0x10 RSP: 0000:ffff95a3417f7ef0 EFLAGS: RAX: a853e2 RBX: a853e6 RCX: ffffffff8bc45d98 rcu_sched detected stalls on CPUs/tasks RDX: RSI: RDI: ffffffff8cf7f34c RBP: ffff95a3417f7f00 R08: fffffffe R09: e R10: R11: a R12: ffff93f7ddfae100 R13: ffff95a3400d7cf0 R14: R15: ffff93f7ddd172c0 FS: (0000) GS:ffff93f7dfc00000(0000) knlgs: CS: 0010 DS: 0000 ES: 0000 CR0: CR2: CR3: a0a000 CR4: f0 Call Trace:? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 10
11 Which CPU is Stalled? (First Format) INFO: rcu_sched detected stalls on CPUs/tasks: : (1 GPs behind) idle=bf2/ /0 softirq=554/555 fqs=6754 (detected by 1, t=21003 jiffies, g=-154, c=-155, q=120339) Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 softirq=554/555 fqs=6754 CPU: 0 PID: 773 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 task: ffff93f7ddd172c0 task.stack: ffff95a3417f4000 RIP: 0010:get_seconds+0xc/0x10 RSP: 0000:ffff95a3417f7ef0 EFLAGS: GP start/now RAX: a853e2 RBX: a853e6 RCX: Idle/offline ffffffff8bc45d98 scans RDX: RSI: RDI: ffffffff8cf7f34c RBP: ffff95a3417f7f00 R08: fffffffe R09: e R10: R11: a R12: ffff93f7ddfae100 idle=bf2/ /0 R13: ffff95a3400d7cf0 R14: R15: ffff93f7ddd172c0 FS: (0000) GS:ffff93f7dfc00000(0000) knlgs: CS: 0010 DS: 0000 ES: 0000 CR0: CPU was aware of last GP CR2: CR3: a0a000 CR4: f0 Call Trace: Interrupts disabled for awhile? rcu_torture_stall+0xcb/0x140 NMI nesting kthread+0x104/0x140 CPU online for next GP begin? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 CPU was online at GP begin ret_from_fork+0x22/0x30 Dyntick counter : (1 GPs behind) 11 CPU is online now Stalled CPU Process/irq nesting
12 Which CPU Detected the Stall? (First Format) INFO: rcu_sched detected stalls on CPUs/tasks: : (1 GPs behind) idle=bf2/ /0 softirq=554/555 fqs=6754 (detected by 1, t=21003 jiffies, g=-154, c=-155, q=120339) Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 773 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 task: ffff93f7ddd172c0 task.stack: ffff95a3417f4000 RIP: 0010:get_seconds+0xc/0x10 RSP: 0000:ffff95a3417f7ef0 EFLAGS: RAX: a853e2 RBX: a853e6 RCX: ffffffff8bc45d98 RDX: RSI: RDI: ffffffff8cf7f34c RBP: ffff95a3417f7f00 R08: fffffffe R09: e R10: R11: a g=-154, R12: ffff93f7ddfae100 c=-155, q= R13: ffff95a3400d7cf0 R14: R15: ffff93f7ddd172c0 FS: (0000) GS:ffff93f7dfc00000(0000) knlgs: CS: 0010 DS: 0000 ES: 0000 CR0: GP duration, jiffies CR2: CR3: a0a000 CR4: f0 Call Trace: CPU detecting stall? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70 GPs # completed? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 GP # started (detected by 1, t=21003 # callbacks queued 12
13 NMI Backtrace Start INFO: rcu_sched detected stalls on CPUs/tasks: : (1 GPs behind) idle=bf2/ /0 softirq=554/555 fqs=6754 (detected by 1, t=21003 jiffies, g=-154, c=-155, q=120339) Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 773 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 task: ffff93f7ddd172c0 task.stack: ffff95a3417f4000 RIP: 0010:get_seconds+0xc/0x10 RSP: 0000:ffff95a3417f7ef0 EFLAGS: RAX: a853e2 RBX: a853e6 RCX: ffffffff8bc45d98 RDX: RSI: RDI: ffffffff8cf7f34c RBP: ffff95a3417f7f00 R08: fffffffe R09: e R10: R11: a R12: ffff93f7ddfae100 R13: ffff95a3400d7cf0 R14: R15: ffff93f7ddd172c0 FS: (0000) GS:ffff93f7dfc00000(0000) knlgs: CS: 0010 DS: 0000 ES: 0000 CR0: CR2: CR3: a0a000 CR4: f0 Call Trace:? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 13
14 NMI Backtrace Stack INFO: rcu_sched detected stalls on CPUs/tasks: : (1 GPs behind) idle=bf2/ /0 softirq=554/555 fqs=6754 (detected by 1, t=21003 jiffies, g=-154, c=-155, q=120339) Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 773 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 task: ffff93f7ddd172c0 task.stack: ffff95a3417f4000 RIP: 0010:get_seconds+0xc/0x10 RSP: 0000:ffff95a3417f7ef0 EFLAGS: rcu_torture_stall RAX: a853e2 RBX: a853e6 RCX: ffffffff8bc45d98 RDX: RSI: RDI: ffffffff8cf7f34c RBP: ffff95a3417f7f00 R08: fffffffe R09: e R10: R11: a R12: ffff93f7ddfae100 R13: ffff95a3400d7cf0 R14: R15: ffff93f7ddd172c0 FS: (0000) GS:ffff93f7dfc00000(0000) knlgs: CS: 0010 DS: 0000 ES: 0000 CR0: CR2: CR3: a0a000 CR4: f0 Call Trace:? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 14
15 Example RCU CPU Stall Warning Splat (2 nd Format) INFO: rcu_sched self-detected stall on CPU 0-...: (20937 ticks this GP) idle=b5e/ /0 softirq=258/258 fqs=5176 (t=21000 jiffies g=-159 c=-160 q=98) NMI backtrace for cpu 0 CPU: 0 PID: 713 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 Call Trace: <IRQ> dump_stack+0x4d/0x6e nmi_cpu_backtrace+0xc5/0xd0... smp_apic_timer_interrupt+0x33/0x50 apic_timer_interrupt+0x86/0x90 RIP: 0010:get_seconds+0x0/0x10 RSP: 0000:ffffa446813ebef0 EFLAGS: ORIG_RAX: ffffffffffffff10 RAX: f5 RBX: f9 RCX: ffffffff84a45cd8 RDX: RSI: RDI: ffffffff85d7320c RBP: ffffa446813ebf00 R08: fffffffe R09: fd R10: R11: a R12: ffff8810de08f0c0 R13: ffffa446800d3cf0 R14: R15: ffff8810de0ea580 </IRQ>? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 15
16 Example RCU CPU Stall Warning Splat (2 nd Format) INFO: rcu_sched self-detected stall on CPU 0-...: (20937 ticks this GP) idle=b5e/ /0 softirq=258/258 fqs=5176 (t=21000 jiffies g=-159 c=-160 q=98) NMI backtrace for cpu 0 CPU: 0 PID: 713 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 Call Trace: <IRQ> dump_stack+0x4d/0x6e nmi_cpu_backtrace+0xc5/0xd0... smp_apic_timer_interrupt+0x33/0x50 apic_timer_interrupt+0x86/0x90 rcu_sched self-detected stall on CPU RIP: 0010:get_seconds+0x0/0x10 RSP: 0000:ffffa446813ebef0 EFLAGS: ORIG_RAX: ffffffffffffff10 RAX: f5 RBX: f9 RCX: ffffffff84a45cd8 RDX: RSI: RDI: ffffffff85d7320c RBP: ffffa446813ebf00 R08: fffffffe R09: fd R10: R11: a R12: ffff8810de08f0c0 R13: ffffa446800d3cf0 R14: R15: ffff8810de0ea580 </IRQ>? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 16
17 Which CPU is Stalled? (2 nd Format) INFO: rcu_sched self-detected stall on CPU 0-...: (20937 ticks this GP) idle=b5e/ /0 softirq=258/258 fqs=5176 (t=21000 jiffies g=-159 c=-160 q=98) NMI backtrace for cpu 0 CPU: 0 PID: 713 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 Call Trace: <IRQ> dump_stack+0x4d/0x6e nmi_cpu_backtrace+0xc5/0xd : (20937 ticks this GP) smp_apic_timer_interrupt+0x33/0x50 apic_timer_interrupt+0x86/0x90 RIP: 0010:get_seconds+0x0/0x10 RSP: 0000:ffffa446813ebef0 EFLAGS: ORIG_RAX: ffffffffffffff10 RAX: f5 RBX: f9 RCX: ffffffff84a45cd8 RDX: RSI: RDI: ffffffff85d7320c RBP: ffffa446813ebf00 R08: fffffffe R09: fd R10: CPU R11: was a aware of last GP R12: ffff8810de08f0c0 R13: ffffa446800d3cf0 R14: R15: ffff8810de0ea580 </IRQ> Interrupts enabled? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140 CPU online for next GP begin? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 CPU was online at GP begin ret_from_fork+0x22/0x30 17 CPU is online now Stalled CPU
18 NMI Backtrace Start Plus Interrupt Stack (2 nd Format) INFO: rcu_sched self-detected stall on CPU 0-...: (20937 ticks this GP) idle=b5e/ /0 softirq=258/258 fqs=5176 (t=21000 jiffies g=-159 c=-160 q=98) NMI backtrace for cpu 0 CPU: 0 PID: 713 Comm: rcu_torture_sta Not tainted rc2+ #1 Hardware name: QEMU Standard PC (i440fx + PIIX, 1996), BIOS Bochs 01/01/2011 Call Trace: <IRQ> dump_stack+0x4d/0x6e nmi_cpu_backtrace+0xc5/0xd0... smp_apic_timer_interrupt+0x33/0x50 rcu_torture_stall apic_timer_interrupt+0x86/0x90 RIP: 0010:get_seconds+0x0/0x10 RSP: 0000:ffffa446813ebef0 EFLAGS: ORIG_RAX: ffffffffffffff10 RAX: f5 RBX: f9 RCX: ffffffff84a45cd8 RDX: RSI: RDI: ffffffff85d7320c RBP: ffffa446813ebf00 R08: fffffffe R09: fd R10: R11: a R12: ffff8810de08f0c0 R13: ffffa446800d3cf0 R14: R15: ffff8810de0ea580 </IRQ>? rcu_torture_stall+0xcb/0x140 kthread+0x104/0x140? rcu_torture_stats+0x70/0x70? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 Ignore interrupt frame 18
19 Repeated Stall Warnings: Compare Stack Traces! At Least 21 Seconds At Least 63 Seconds CPU 0 RCU Reader (Including Interrupts Disabled) CPU 1 RCU Reader Userspace Execution RCU Stall Warning #1 Stall Warning #2 Time 19
20 What Causes CPU Stalls? 20
21 What Causes CPU Stalls? rcu_read_lock(); for (;;) do_something; rcu_read_unlock(); How to fix this? 21
22 What Causes CPU Stalls? rcu_read_lock(); for (;;) do_something; rcu_read_unlock(); How to fix this? Break out of RCU read-side critical section occasionally Preferably every few milliseconds Ensure that your loops are finite 22
23 What Causes CPU Stalls? local_irq_disable(); for (;;) do_something; local_irq_enable(); How to fix this? 23
24 What Causes CPU Stalls? local_irq_disable(); for (;;) do_something; local_irq_enable(); How to fix this? Break out of interrupt-disable regions occasionally Increase break-out frequency until tglx stops throwing frozen sharks Ensure that your loops are finite Decrease loop length until tglx stops throwing frozen sharks 24
25 Variations on Long-Reader and IRQ-Disable Themes Looping with preemption disabled You should expect incoming sharks in this case as well Looping with bottom-half execution disabled Ditto Long-running interrupt within RCU reader PREEMPT=n: Looping without invoking schedule() or cond_resched_rcu_qs() PREEMPT=y: Indefinitely preempting an RCU reader Can also try RCU_BOOST=y 25
26 What Causes CPU Stalls? Embedded Computer System 115Kbaud High-Speed Serial Line!!! 26
27 What Causes CPU Stalls? Embedded Computer System 115Kbaud High-Speed Serial Line!!! Unless you have 500K console output... 27
28 What Causes CPU Stalls? Develop on fast system that just barely avoids CPU stalls Then deploy on slow system Interrupt overload Turning on super-high-overhead debugging So adjust the CPU stall timeout in this case!!! Prevent RCU_SOFTIRQ from running For example, CPU-bound high-priority real-time process Completely shut off CPU's scheduler-clock interrupt Hardware failure In one case, a fail-stop CPU! Timer issues are a recurrent theme 28
29 RCU Bugs Can Also Cause CPU Stalls When things are stuck for 21 seconds, no need to be dainty False!!! As I spent a couple years learning... Stall-warning messages can cause the stall to end After part of the message is printed... This case now flagged Extremely quiet embedded systems have their own issues They can enter states noisy systems avoid!!! RCU kthread wakeup failures 29
30 Evolution of RCU Grace-Period Handling Pre-v3.8: RCU state machine driven via softirq 30
31 First Clue of Large-System RT-Response Importance 31 wikimedia.org
32 Evolution of RCU Grace-Period Handling Pre-v3.8: RCU state machine driven via softirq v3.8: RCU state machine driven via kthread 32
33 Evolution of RCU Grace-Period Handling Pre-v3.8: RCU state machine driven via softirq v3.8: RCU state machine driven via kthread 33
34 RCU Grace-Period Kernel Thread Wakeup Failures rcu_bh kthread starved for jiffies! g c f0x0 RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=0 rcu_bh R running task x Call Trace: schedule+0x20b/0x6c0 schedule+0x31/0x80 schedule_timeout+0x170/0x2f0? call_timer_fn+0x130/0x130 rcu_gp_kthread+0x4be/0xd90 kthread+0x104/0x140? rcu_oom_notify+0xf0/0xf0? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 34
35 RCU Grace-Period Kernel Thread Wakeup Failures rcu_bh kthread starved for jiffies! g c f0x0 RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=0 rcu_bh R running task x Call Trace: schedule+0x20b/0x6c0 schedule+0x31/0x80 schedule_timeout+0x170/0x2f0? call_timer_fn+0x130/0x130 rcu_gp_kthread+0x4be/0xd90 kthread+0x104/0x140? rcu_oom_notify+0xf0/0xf0? kthread_park+0x60/0x60 ret_from_fork+0x22/0x30 rcu_bh kthread starved for jiffies 35 RCU cannot do much for you if you don't let its kthreads run!!!
36 Obligatory War Stories 36
37 My Favorite? CPU-0 Standard Time 37
38 CPU-0 Standard Time CPU 0 CPU 1 CPU 2 Time
39 CPU-0 Standard Time GP Start, t= 0 CPU 0 CPU stall, t=30 CPU 1 CPU 2 Time
40 Time Waits For No One, But... 40
41 High-Level RCU Grace-Period Processing Initialize Grace Period No Wait A Few Jiffies Wait For New Grace-Period Request Grace Period Done? Yes Check Idle CPUs Clean Up Grace Period 41
42 High-Level Timer Processing RCU GP kthread: post timer on timer wheel In the fullness of time... raise_softirq of TIMER_SOFTIRQ Wake up requesting process 42
43 High-Level Timer Processing, CPU Offline RCU GP kthread: post timer on CPU 5 timer wheel CPU 5 goes offline In the fullness of time... Timer migrated to surviving CPU raise_softirq of TIMER_SOFTIRQ Wake up requesting process 43
44 High-Level Timer Processing, CPU Offline, RCU RCU GP kthread: post timer on CPU 5 timer wheel CPU 5 goes offline In the fullness of time... Hotplug notifier waits for RCU grace period raise_softirq of TIMER_SOFTIRQ Timer migrated to surviving CPU Wake up requesting process 44
45 Time Waits For No One, But It Can Deadlock With CPU-Hotplug Offline and RCU Grace Periods!!! 45
46 Summary 46
47 Summary RCU CPU stall warnings are a valuable diagnostic tool CPUs stuck in various unhelpful states Extreme overload Priority issues Temporal anomalies Low-level software issues Hardware problems RCU bugs Prevention: Pause points in unbounded loops Test on deployment-class systems (or adjust CPU-stall timeout) Assign priorities carefully Respect the passage of time 47
48 Summary RCU CPU stall warnings are a valuable diagnostic tool CPUs stuck in various unhelpful states Extreme overload Priority issues Temporal anomalies Low-level software issues Hardware problems RCU bugs Prevention: Pause points in unbounded loops Test on deployment-class systems (or adjust CPU-stall timeout) Assign priorities carefully Respect the passage of time Make sure this McKenney fellow doesn't mess up RCU! 48
49 Legal Statement This work represents the view of the author and does not necessarily represent the view of IBM. IBM and IBM (logo) are trademarks or registered trademarks of International Business Machines Corporation in the United States and/or other countries. Linux is a registered trademark of Linus Torvalds. Other company, product, and service names may be trademarks or service marks of others. 49
50 Questions? 50
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