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@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()); > } 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@xxxxxxxxxxxxxxxxx> ---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()); } //////////////////////////////////////////////////////////////////////////////