[PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()
From: Huacai Chen
Date: Tue Aug 22 2023 - 00:03:17 EST
The KGDB initial breakpoint gets an rcu stall warning after commit
a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in
rcu_cpu_stall_reset()").
[ 53.452051] rcu: INFO: rcu_preempt self-detected stall on CPU
[ 53.487950] rcu: 3-...0: (1 ticks this GP) idle=0e2c/1/0x4000000000000000 softirq=375/375 fqs=8
[ 53.528243] rcu: (t=12297 jiffies g=-995 q=1 ncpus=4)
[ 53.564840] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc2+ #4848
[ 53.603005] Hardware name: Loongson Loongson-3A5000-HV-7A2000-1w-V0.1-CRB/Loongson-LS3A5000-7A2000-1w-CRB-V1.21, BIOS Loongson-UDK2018-V2.0.05099-beta8 08
[ 53.682062] pc 9000000000332100 ra 90000000003320f4 tp 90000001000a0000 sp 90000001000a3710
[ 53.724934] a0 9000000001d4b488 a1 0000000000000000 a2 0000000000000001 a3 0000000000000000
[ 53.768179] a4 9000000001d526c8 a5 90000001000a38f0 a6 000000000000002c a7 0000000000000000
[ 53.810751] t0 00000000000002b0 t1 0000000000000004 t2 900000000131c9c0 t3 fffffffffffffffa
[ 53.853249] t4 0000000000000080 t5 90000001002ac190 t6 0000000000000004 t7 9000000001912d58
[ 53.895684] t8 0000000000000000 u0 90000000013141a0 s9 0000000000000028 s0 9000000001d512f0
[ 53.937633] s1 9000000001d51278 s2 90000001000a3798 s3 90000000019fc410 s4 9000000001d4b488
[ 53.979486] s5 9000000001d512f0 s6 90000000013141a0 s7 0000000000000078 s8 9000000001d4b450
[ 54.021175] ra: 90000000003320f4 kgdb_cpu_enter+0x534/0x640
[ 54.060150] ERA: 9000000000332100 kgdb_cpu_enter+0x540/0x640
[ 54.098347] CRMD: 000000b0 (PLV0 -IE -DA +PG DACF=CC DACM=CC -WE)
[ 54.136621] PRMD: 0000000c (PPLV0 +PIE +PWE)
[ 54.172192] EUEN: 00000000 (-FPE -SXE -ASXE -BTE)
[ 54.207838] ECFG: 00071c1c (LIE=2-4,10-12 VS=7)
[ 54.242503] ESTAT: 00000800 [INT] (IS=11 ECode=0 EsubCode=0)
[ 54.277996] PRID: 0014c011 (Loongson-64bit, Loongson-3A5000-HV)
[ 54.313544] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc2+ #4848
[ 54.430170] Stack : 0072617764726148 0000000000000000 9000000000223504 90000001000a0000
[ 54.472308] 9000000100073a90 9000000100073a98 0000000000000000 9000000100073bd8
[ 54.514413] 9000000100073bd0 9000000100073bd0 9000000100073a00 0000000000000001
[ 54.556018] 0000000000000001 9000000100073a98 99828271f24e961a 90000001002810c0
[ 54.596924] 0000000000000001 0000000000010003 0000000000000000 0000000000000001
[ 54.637115] ffff8000337cdb80 0000000000000001 0000000006360000 900000000131c9c0
[ 54.677049] 0000000000000000 0000000000000000 90000000017b4c98 9000000001912000
[ 54.716394] 9000000001912f68 9000000001913000 9000000001912f70 00000000000002b0
[ 54.754880] 90000000014a8840 0000000000000000 900000000022351c 0000000000000000
[ 54.792372] 00000000000002b0 000000000000000c 0000000000000000 0000000000071c1c
[ 54.829302] ...
[ 54.859163] Call Trace:
[ 54.859165] [<900000000022351c>] show_stack+0x5c/0x180
[ 54.918298] [<90000000012f6100>] dump_stack_lvl+0x60/0x88
[ 54.949251] [<90000000012dd5d8>] rcu_dump_cpu_stacks+0xf0/0x148
[ 54.981116] [<90000000002d2fb8>] rcu_sched_clock_irq+0xb78/0xe60
[ 55.012744] [<90000000002e47cc>] update_process_times+0x6c/0xc0
[ 55.044169] [<90000000002f65d4>] tick_sched_timer+0x54/0x100
[ 55.075488] [<90000000002e5174>] __hrtimer_run_queues+0x154/0x240
[ 55.107347] [<90000000002e6288>] hrtimer_interrupt+0x108/0x2a0
[ 55.139112] [<9000000000226418>] constant_timer_interrupt+0x38/0x60
[ 55.170749] [<90000000002b3010>] __handle_irq_event_percpu+0x50/0x160
[ 55.203141] [<90000000002b3138>] handle_irq_event_percpu+0x18/0x80
[ 55.235064] [<90000000002b9d54>] handle_percpu_irq+0x54/0xa0
[ 55.266241] [<90000000002b2168>] generic_handle_domain_irq+0x28/0x40
[ 55.298466] [<9000000000aba95c>] handle_cpu_irq+0x5c/0xa0
[ 55.329749] [<90000000012f7270>] handle_loongarch_irq+0x30/0x60
[ 55.361476] [<90000000012f733c>] do_vint+0x9c/0x100
[ 55.391737] [<9000000000332100>] kgdb_cpu_enter+0x540/0x640
[ 55.422440] [<9000000000332b64>] kgdb_handle_exception+0x104/0x180
[ 55.452911] [<9000000000232478>] kgdb_loongarch_notify+0x38/0xa0
[ 55.481964] [<900000000026b4d4>] notify_die+0x94/0x100
[ 55.509184] [<90000000012f685c>] do_bp+0x21c/0x340
[ 55.562475] [<90000000003315b8>] kgdb_compiled_break+0x0/0x28
[ 55.590319] [<9000000000332e80>] kgdb_register_io_module+0x160/0x1c0
[ 55.618901] [<9000000000c0f514>] configure_kgdboc+0x154/0x1c0
[ 55.647034] [<9000000000c0f5e0>] kgdboc_probe+0x60/0x80
[ 55.674647] [<9000000000c96da8>] platform_probe+0x68/0x100
[ 55.702613] [<9000000000c938e0>] really_probe+0xc0/0x340
[ 55.730528] [<9000000000c93be4>] __driver_probe_device+0x84/0x140
[ 55.759615] [<9000000000c93cdc>] driver_probe_device+0x3c/0x120
[ 55.787990] [<9000000000c93e8c>] __device_attach_driver+0xcc/0x160
[ 55.817145] [<9000000000c91290>] bus_for_each_drv+0x90/0x100
[ 55.845654] [<9000000000c94328>] __device_attach+0xa8/0x1a0
[ 55.874145] [<9000000000c925f0>] bus_probe_device+0xb0/0xe0
[ 55.902572] [<9000000000c8ec7c>] device_add+0x65c/0x860
[ 55.930635] [<9000000000c96704>] platform_device_add+0x124/0x2c0
[ 55.959669] [<9000000001452b38>] init_kgdboc+0x58/0xa0
[ 55.987677] [<900000000022015c>] do_one_initcall+0x7c/0x1e0
[ 56.016134] [<9000000001420f1c>] kernel_init_freeable+0x22c/0x2a0
[ 56.045128] [<90000000012f923c>] kernel_init+0x20/0x124
Currently rcu_cpu_stall_reset() set rcu_state.jiffies_stall to one check
period later, i.e. jiffies + rcu_jiffies_till_stall_check(). But jiffies
is only updated in the timer interrupt, so when kgdb_cpu_enter() begins
to run there may already be nearly one rcu check period after jiffies.
Since all interrupts are disabled during kgdb_cpu_enter(), jiffies will
not be updated. When kgdb_cpu_enter() returns, rcu_state.jiffies_stall
maybe already gets timeout.
We can set rcu_state.jiffies_stall to two rcu check periods later, e.g.
jiffies + (rcu_jiffies_till_stall_check() * 2) in rcu_cpu_stall_reset()
to avoid this problem. But this isn't a complete solution because kgdb
may take a very long time in irq disabled context.
Instead, update jiffies at the beginning of rcu_cpu_stall_reset() can
solve all kinds of problems [1]. But this causes a new problem because
updating jiffies is not NMI safe while rcu_cpu_stall_reset() may be used
in NMI context.
So we don't update the global jiffies, but only add the time 'delta' to
jiffies locally at the beginning of rcu_cpu_stall_reset() which has the
same effect.
[1] https://lore.kernel.org/rcu/20230814020045.51950-1-chenhuacai@xxxxxxxxxxx/T/#t
Cc: stable@xxxxxxxxxxxxxxx
Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
Reported-off-by: Binbin Zhou <zhoubinbin@xxxxxxxxxxx>
Signed-off-by: Huacai Chen <chenhuacai@xxxxxxxxxxx>
---
V2: Use NMI safe functions.
V3: Add comments to explain why.
kernel/rcu/tree_stall.h | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b10b8349bb2a..e4e53113d062 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -150,11 +150,26 @@ static void panic_on_rcu_stall(void)
* rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
*
* The caller must disable hard irqs.
+ *
+ * The jiffies updating may be delayed for a very long time due to tickless and
+ * irq disabled, especially in the KGDB case, so we need to add the delayed time
+ * (delta) to rcu_state.jiffies_stall.
+ *
+ * This function may be called in NMI context, so we cannot use ktime_get_ns()
+ * and ktime_get_coarse_ns(). Instead, we use their inaccurate but safe friends
+ * ktime_get_mono_fast_ns() and ktime_get_seconds() which will cause rcu_state.
+ * jiffies_stall to be a little large than expected (harmless and safer).
*/
void rcu_cpu_stall_reset(void)
{
+ u64 curr, last, delta;
+
+ curr = ktime_get_mono_fast_ns();
+ last = ktime_get_seconds() * NSEC_PER_SEC;
+ delta = nsecs_to_jiffies(curr - last);
+
WRITE_ONCE(rcu_state.jiffies_stall,
- jiffies + rcu_jiffies_till_stall_check());
+ jiffies + delta + rcu_jiffies_till_stall_check());
}
//////////////////////////////////////////////////////////////////////////////
--
2.39.3