Hi, Alan, On Thu, Aug 17, 2023 at 12:52 AM Alan Huang <mmpgouride@xxxxxxxxx> wrote: > > >> > >>>>>>>>>>> > >>>>>>>>>>> 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? > > This can solve the kgdb problem, but I found that most callers of > > rcu_cpu_stall_reset() are in irq disabled context so they may meet > > The duration of other contexts where interrupts are disabled may not be as long as in the case of kgdb？ > > > similar problems. Modifying rcu_cpu_stall_reset() can solve all of > > them. > > > > But due to the NMI issue, from my point of view, setting jiffies_stall > > to jiffies + 300*HZ is the best solution now. :) > > If I understand correctly, the NMI issue is the deadlock issue? If so, plus the short duration of other irq disabled > contexts, it’s ok just update jiffies in dbg_touch_watchdogs. > > Please correct me if anything wrong. :) The timeout value can be configured as short as 3 seconds, in this case other callers may also have problems. Huacai > > > > > Huacai > >> > >>> 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 > >
