>>>>>>>>> >>>>>>>>> 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. >>> >> >> In do_update_jiffies_64(), need to acquire jiffies_lock raw spinlock, >> like you said, kgdb is not the only caller of rcu_cpu_stall_reset(), >> the rcu_cpu_stall_reset() maybe invoke in NMI (arch/x86/platform/uv/uv_nmi.c) > Reset rcu_cpu_stall_suppress in rcu_gp_init()/rcu_gp_cleanup() is > still not so good to me, because it does a useless operation in most > cases. Moreover, the rcu core is refactored again and again, something > may be changed in future. > > If do_update_jiffies_64() cannot be used in NMI context, can we What about updating jiffies in dbg_touch_watchdogs or adding a wrapper which updates both jiffies and jiffies_stall? > consider my old method [1]? > https://lore.kernel.org/rcu/CAAhV-H7j9Y=VvRLm8thLw-EX1PGqBA9YfT4G1AN7ucYS=iP+DQ@xxxxxxxxxxxxxx/T/#t > > Of course we should set rcu_state.jiffies_stall large enough, so we > can do like this: > > void rcu_cpu_stall_reset(void) > { > WRITE_ONCE(rcu_state.jiffies_stall, > - jiffies + rcu_jiffies_till_stall_check()); > + jiffies + 300 * HZ); > } > > 300s is the largest timeout value, and I think 300s is enough here in practice. > > Huacai > >> >> Thanks >> Zqiang >> >> >>> 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