[PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Huacai Chen]

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@loongson.cn/T/#t

Cc: stable@vger.kernel.org
Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
Reported-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
---
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

Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Joel Fernandes]

On Tue, Aug 22, 2023 at 12:02:48PM +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
[...]
> 
> [1] https://lore.kernel.org/rcu/20230814020045.51950-1-chenhuacai@loongson.cn/T/#t
> 
> Cc: stable@vger.kernel.org
> Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
> Reported-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---
> 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());
>  }

I prefer the following diff on top of your patch to take advantage of UBSAN
detecting overflows.

If you take my diff, feel free to add:
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---8<-----------------------

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5e9e4779bdf1..3398cf2d19c5 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -162,14 +162,15 @@ static void panic_on_rcu_stall(void)
  */
 void rcu_cpu_stall_reset(void)
 {
-	u64 curr, last, delta;
+	ktime_t last, delta_ns;
+	u64 delta_jiff;
 
-	curr = ktime_get_mono_fast_ns();
 	last = ktime_get_seconds() * NSEC_PER_SEC;
-	delta = nsecs_to_jiffies(curr - last);
+	delta_ns = ktime_sub(ktime_get_mono_fast_ns(), last);
+	delta_jiff = nsecs_to_jiffies(delta_ns);
 
 	WRITE_ONCE(rcu_state.jiffies_stall,
-		   jiffies + delta + rcu_jiffies_till_stall_check());
+		   jiffies + delta_jiff + rcu_jiffies_till_stall_check());
 }
 
 //////////////////////////////////////////////////////////////////////////////

Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Joel,

On Tue, Aug 22, 2023 at 11:34 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Tue, Aug 22, 2023 at 12:02:48PM +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
> [...]
> >
> > [1] https://lore.kernel.org/rcu/20230814020045.51950-1-chenhuacai@loongson.cn/T/#t
> >
> > Cc: stable@vger.kernel.org
> > Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
> > Reported-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
> > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> > ---
> > 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());
> >  }
>
> I prefer the following diff on top of your patch to take advantage of UBSAN
> detecting overflows.
>
> If you take my diff, feel free to add:
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> ---8<-----------------------
>
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 5e9e4779bdf1..3398cf2d19c5 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -162,14 +162,15 @@ static void panic_on_rcu_stall(void)
>   */
>  void rcu_cpu_stall_reset(void)
>  {
> -       u64 curr, last, delta;
> +       ktime_t last, delta_ns;
> +       u64 delta_jiff;
>
> -       curr = ktime_get_mono_fast_ns();
>         last = ktime_get_seconds() * NSEC_PER_SEC;
> -       delta = nsecs_to_jiffies(curr - last);
> +       delta_ns = ktime_sub(ktime_get_mono_fast_ns(), last);
Though ktime_t is the same as s64/u64 now, but I think we'd better to
not mix them. Then, ktime_get() and ktime_get_coarse() return ktime_t;
ktime_get_ns(), ktime_get_coarse_ns() and ktime_get_mono_fast_ns()
return s64/u64 (means nanoseconds); ktime_get_seconds() returns
seconds but ktime_get_seconds() * NSEC_PER_SEC is also nanoseconds.
So, the type definition in your diff is not suitable,
ktime_sub(ktime_get_mono_fast_ns(), last) is not suitable too.

Huacai

> +       delta_jiff = nsecs_to_jiffies(delta_ns);
>
>         WRITE_ONCE(rcu_state.jiffies_stall,
> -                  jiffies + delta + rcu_jiffies_till_stall_check());
> +                  jiffies + delta_jiff + rcu_jiffies_till_stall_check());
>  }
>
>  //////////////////////////////////////////////////////////////////////////////

Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Joel Fernandes]

On Tue, Aug 22, 2023 at 11:48 AM Huacai Chen <chenhuacai@kernel.org> wrote:
>
> Hi, Joel,
>
> On Tue, Aug 22, 2023 at 11:34 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Tue, Aug 22, 2023 at 12:02:48PM +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
> > [...]
> > >
> > > [1] https://lore.kernel.org/rcu/20230814020045.51950-1-chenhuacai@loongson.cn/T/#t
> > >
> > > Cc: stable@vger.kernel.org
> > > Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
> > > Reported-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
> > > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> > > ---
> > > 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());
> > >  }
> >
> > I prefer the following diff on top of your patch to take advantage of UBSAN
> > detecting overflows.
> >
> > If you take my diff, feel free to add:
> > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> >
> > ---8<-----------------------
> >
> > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > index 5e9e4779bdf1..3398cf2d19c5 100644
> > --- a/kernel/rcu/tree_stall.h
> > +++ b/kernel/rcu/tree_stall.h
> > @@ -162,14 +162,15 @@ static void panic_on_rcu_stall(void)
> >   */
> >  void rcu_cpu_stall_reset(void)
> >  {
> > -       u64 curr, last, delta;
> > +       ktime_t last, delta_ns;
> > +       u64 delta_jiff;
> >
> > -       curr = ktime_get_mono_fast_ns();
> >         last = ktime_get_seconds() * NSEC_PER_SEC;
> > -       delta = nsecs_to_jiffies(curr - last);
> > +       delta_ns = ktime_sub(ktime_get_mono_fast_ns(), last);
> Though ktime_t is the same as s64/u64 now,

No, you are incorrect.  ktime_t is a signed int not u64. I prefer to
use types designed for a specific purpose where possible. It makes the
code safer.

> but I think we'd better to
> not mix them. Then, ktime_get() and ktime_get_coarse() return ktime_t;
> ktime_get_ns(), ktime_get_coarse_ns() and ktime_get_mono_fast_ns()
> return s64/u64 (means nanoseconds); ktime_get_seconds() returns
> seconds but ktime_get_seconds() * NSEC_PER_SEC is also nanoseconds.
> So, the type definition in your diff is not suitable,
> ktime_sub(ktime_get_mono_fast_ns(), last) is not suitable too.
>

Is there a technical reason for your concern? ktime_get_mono_fast_ns()
is assigned to ktime_t in other places. I am not seeing what the
problem is.

Thanks.

Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Huacai Chen]

On Tue, Aug 22, 2023 at 11:58 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Tue, Aug 22, 2023 at 11:48 AM Huacai Chen <chenhuacai@kernel.org> wrote:
> >
> > Hi, Joel,
> >
> > On Tue, Aug 22, 2023 at 11:34 PM Joel Fernandes <joel@joelfernandes.org> wrote:
> > >
> > > On Tue, Aug 22, 2023 at 12:02:48PM +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
> > > [...]
> > > >
> > > > [1] https://lore.kernel.org/rcu/20230814020045.51950-1-chenhuacai@loongson.cn/T/#t
> > > >
> > > > Cc: stable@vger.kernel.org
> > > > Fixes: a80be428fbc1f1f3bc9ed924 ("rcu: Do not disable GP stall detection in rcu_cpu_stall_reset()")
> > > > Reported-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
> > > > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> > > > ---
> > > > 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());
> > > >  }
> > >
> > > I prefer the following diff on top of your patch to take advantage of UBSAN
> > > detecting overflows.
> > >
> > > If you take my diff, feel free to add:
> > > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > >
> > > ---8<-----------------------
> > >
> > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > > index 5e9e4779bdf1..3398cf2d19c5 100644
> > > --- a/kernel/rcu/tree_stall.h
> > > +++ b/kernel/rcu/tree_stall.h
> > > @@ -162,14 +162,15 @@ static void panic_on_rcu_stall(void)
> > >   */
> > >  void rcu_cpu_stall_reset(void)
> > >  {
> > > -       u64 curr, last, delta;
> > > +       ktime_t last, delta_ns;
> > > +       u64 delta_jiff;
> > >
> > > -       curr = ktime_get_mono_fast_ns();
> > >         last = ktime_get_seconds() * NSEC_PER_SEC;
> > > -       delta = nsecs_to_jiffies(curr - last);
> > > +       delta_ns = ktime_sub(ktime_get_mono_fast_ns(), last);
> > Though ktime_t is the same as s64/u64 now,
>
> No, you are incorrect.  ktime_t is a signed int not u64. I prefer to
> use types designed for a specific purpose where possible. It makes the
> code safer.
Yes ktime_t is s64 now, I said s64/u64 because there is some existing
code to mix them (of course not very good).

>
> > but I think we'd better to
> > not mix them. Then, ktime_get() and ktime_get_coarse() return ktime_t;
> > ktime_get_ns(), ktime_get_coarse_ns() and ktime_get_mono_fast_ns()
> > return s64/u64 (means nanoseconds); ktime_get_seconds() returns
> > seconds but ktime_get_seconds() * NSEC_PER_SEC is also nanoseconds.
> > So, the type definition in your diff is not suitable,
> > ktime_sub(ktime_get_mono_fast_ns(), last) is not suitable too.
> >
>
> Is there a technical reason for your concern? ktime_get_mono_fast_ns()
> is assigned to ktime_t in other places. I am not seeing what the
> problem is.
My concern is ktime_t may not always be s64 in future, so there are
ktime_to_ns() and some other similar APIs to convert from one to
another.

Huacai
>
> Thanks.

Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Thomas Gleixner]

On Tue, Aug 22 2023 at 12:02, Huacai Chen wrote:
> + * 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).

What's inaccurate about ktime_get_mono_fast_ns()? Bogus comments are
even worse than no comments.

>   */
>  void rcu_cpu_stall_reset(void)
>  {
> +	u64 curr, last, delta;
> +
> +	curr = ktime_get_mono_fast_ns();
> +	last = ktime_get_seconds() * NSEC_PER_SEC;

So this will trigger a warning when someone debugs suspend with KGDB.

It seems the approach taken here seems to be to throw stuff at the wall
and see what sticks.

Thanks,

        tglx

Re: [PATCH V3] rcu: Update jiffies locally in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Thomas,

On Thu, Aug 24, 2023 at 5:37 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Tue, Aug 22 2023 at 12:02, Huacai Chen wrote:
> > + * 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).
>
> What's inaccurate about ktime_get_mono_fast_ns()? Bogus comments are
> even worse than no comments.
ktime_get_mono_fast_ns() is not as accurate as ktime_get_ns(), I get
this conclusion from:

 * So if the update lowers the slope, readers who are forced to the
 * not yet updated second array are still using the old steeper slope.

>
> >   */
> >  void rcu_cpu_stall_reset(void)
> >  {
> > +     u64 curr, last, delta;
> > +
> > +     curr = ktime_get_mono_fast_ns();
> > +     last = ktime_get_seconds() * NSEC_PER_SEC;
>
> So this will trigger a warning when someone debugs suspend with KGDB.
Yes, ktime_get_seconds() may cause a warning, I haven't noticed this before.

>
> It seems the approach taken here seems to be to throw stuff at the wall
> and see what sticks.
I don't understand what's meaning, but I believe your advice in
another thread is the best solution, so let me try.

Huacai

>
> Thanks,
>
>         tglx