Re: [PATCH V2] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Joel,

On Tue, Aug 22, 2023 at 12:46 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 21, 2023 at 12:16 AM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote:
> >
> > Hi, Joel,
> >
> > On Mon, Aug 21, 2023 at 11:08 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > Hi Huacai,
> > >
> > > On Sun, Aug 20, 2023 at 7:19 AM Huacai Chen <chenhuacai@xxxxxxxxxxx> 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 [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.
> > > >
> > > >  kernel/rcu/tree_stall.h | 8 +++++++-
> > > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > > > index b10b8349bb2a..605a774241c8 100644
> > > > --- a/kernel/rcu/tree_stall.h
> > > > +++ b/kernel/rcu/tree_stall.h
> > > > @@ -153,8 +153,14 @@ static void panic_on_rcu_stall(void)
> > > >   */
> > > >  void rcu_cpu_stall_reset(void)
> > > >  {
> > > > +       u64 curr, last, delta;
> > > > +
> > > > +       curr = ktime_get_mono_fast_ns();
> > > > +       last = ktime_get_seconds() * NSEC_PER_SEC;
> > >
> > > Heh, so if jiffies are correctly updated in the first place, then this
> > > code will now inject errors up to 1 second? Example delta=0.99s when
> > > it should really be 0 if jiffies updated correctly.
> > Yes, the jiffies_stall may be a little larger than expected, but this
> > means safer, and doesn't cause problems.
>
> True, but I think you need to add a comment here about the potential
> inaccuracies including the fact that, even when jiffies is updating
> normally, there are still some additional inaccuracies added.
>
> But I agree with you that it is better than not doing anyway and we
> need something here so I am OK as such with your solution.
>
> May I make a suggestion for additional safety / guard-rails: Please
> cap the computed value delta in case it is some negative number or
> above a certain threshold. That way we don't inadvertently set
> jiffies_stall to something ridiculous.
>
> > > > +       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());
> > > >  }
> > > >
> > >
> > > Also I think this logic should be special-cased only for KGDB and not
> > > for all users since the disabling of IRQs for such a long time is kgdb
> > > specific.
> > >
> > > Ideally you'd want to measure the time in nanoseconds for which IRQ is
> > > disabled in kgdb, and the supply that delta to rcu_cpu_stall_reset().
> > > Something like:
> > >
> > > 1. Sample  ktime_get_mono_fast_ns() and jiffies just after the
> > > beginning of the IRQ disabled section.
> > > 2. Sample  ktime_get_mono_fast_ns() and jiffies just before the end of
> > > the IRQ disabled section.
> > > 3. Calculate if jiffies did not change in the meanwhile (old jiffies
> > > == new jiffies) but the old and new ktimes show that the time shifted.
> > > 4. If #3 is true, then take the difference between the old and new
> > > ktime_get_mono_fast_ns().
> > > 5. Pass the delta from #4 to rcu_cpu_stall_reset() from kgdb, pass 0
> > > from all other callers. Add it to jiffies to compensate.
> > >
> > > Not sure if that'll work or if there's a better solution.
> > >
> > > Or perhaps we should do your patch above but make it KGDB-specific.
> > > That's better than magic numbers and better than affecting all
> > > non-KGDB users.
> > This makes the solution too complicated, and I don't think non-KGDB
> > users completely don't need this. Yes, the "irq disabled time" for
> > them is much shorter than KGDB, but for a tickless system, the
> > "jiffies delay time" may be much longer than the "irq disabled time".
> > The "jiffies delay time" will cause jiffies_stall to be smaller than
> > expected, and then cause a stall warning.
>
> I tend to agree with you. I only worry that we should not regress or
> affect cases that no one has complained about. However, as long as you
> are adding a comment, I am OK with it unless others disagree due to
> some reason.
OK, I will add some comments to make things clear.

Huacai
>
> thanks,
>
>  - Joel