Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Qiang,

On Wed, Aug 16, 2023 at 1:09 PM Z qiang <qiang.zhang1211@xxxxxxxxx> wrote:
>
> >
> > Hi, Qiang,
> >
> > On Wed, Aug 16, 2023 at 11:16 AM Z qiang <qiang.zhang1211@xxxxxxxxx> wrote:
> > >
> > > >
> > > > Hi, Paul,
> > > >
> > > > On Tue, Aug 15, 2023 at 12:15 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, Aug 14, 2023 at 10:00:45AM +0800, Huacai Chen wrote:
> > > > > > 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.
> > > > >
> > > > > Would it make sense for there to be a kgdb_cpu_exit()?  In that case,
> > > > > the stalls could simply be suppressed at the beginning of the debug
> > > > > session and re-enabled upon exit, as is currently done for sysrq output
> > > > > via rcu_sysrq_start() and rcu_sysrq_end().
> > > > Thank you for your advice, but that doesn't help. Because
> > > > rcu_sysrq_start() and rcu_sysrq_end() try to suppress the warnings
> > > > during sysrq, but kgdb already has no warnings during kgdb_cpu_enter()
> > > > since it is executed in irq disabled context. Instead, this patch
> > > > wants to suppress the warnings *after* kgdb_cpu_enter() due to a very
> > > > old jiffies value.
> > > >
> > >
> > > Hello, Huacai
> > >
> > > Is it possible to set  the rcu_cpu_stall_suppress is true in
> > > dbg_touch_watchdogs()
> > > and reset the rcu_cpu_stall_suppress at the beginning and end of the
> > > RCU grace period?
> > This is possible but not the best: 1, kgdb is not the only caller of
> > rcu_cpu_stall_reset(); 2, it is difficult to find the "end" to reset
> > rcu_cpu_stall_suppress.
> >
>
> You can replace rcu_state.jiffies_stall update by setting rcu_cpu_stall_suppress
> in rcu_cpu_stall_reset(),  and reset rcu_cpu_stall_suppress in rcu_gp_init() and
> rcu_gp_cleanup().
What's the advantage compared with updating jiffies? Updating jiffies
seems more straight forward.

Huacai

>
> Thanks
> Zqiang
>
> >
> > > or set rcupdate.rcu_cpu_stall_suppress_at_boot=1 in bootargs can
> > > suppress RCU stall
> > > in booting.
> > This is also possible, but it suppresses all kinds of stall warnings,
> > which is not what we want.
> >
> > Huacai
> > >
> > >
> > > Thanks
> > > Zqiang
> > >
> > >
> > > >
> > > > Huacai
> > > >
> > > > >
> > > > >                                                         Thanx, Paul
> > > > >
> > > > > > 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>
> > > > > > ---
> > > > > >  kernel/rcu/tree_stall.h | 1 +
> > > > > >  1 file changed, 1 insertion(+)
> > > > > >
> > > > > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > > > > > index b10b8349bb2a..1c7b540985bf 100644
> > > > > > --- a/kernel/rcu/tree_stall.h
> > > > > > +++ b/kernel/rcu/tree_stall.h
> > > > > > @@ -153,6 +153,7 @@ static void panic_on_rcu_stall(void)
> > > > > >   */
> > > > > >  void rcu_cpu_stall_reset(void)
> > > > > >  {
> > > > > > +     do_update_jiffies_64(ktime_get());
> > > > > >       WRITE_ONCE(rcu_state.jiffies_stall,
> > > > > >                  jiffies + rcu_jiffies_till_stall_check());
> > > > > >  }
> > > > > > --
> > > > > > 2.39.3
> > > > > >