On Thu, Aug 24, 2023 at 12:03:25AM +0200, Thomas Gleixner wrote: > On Thu, Aug 17 2023 at 16:06, Huacai Chen wrote: > > On Thu, Aug 17, 2023 at 3:27 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > >> > If do_update_jiffies_64() cannot be used in NMI context, > >> > >> Can you not make the jiffies update conditional on whether it is > >> called within NMI context? > > Which solves what? If KGDB has a breakpoint in the jiffies lock held > region then you still dead lock. > > >> I dislike that.. > > Is this acceptable? > > > > void rcu_cpu_stall_reset(void) > > { > > unsigned long delta; > > > > delta = nsecs_to_jiffies(ktime_get_ns() - ktime_get_coarse_ns()); > > > > WRITE_ONCE(rcu_state.jiffies_stall, > > jiffies + delta + rcu_jiffies_till_stall_check()); > > } > > > > This can update jiffies_stall without updating jiffies (but has the > > same effect). > > Now you traded the potential dead lock on jiffies lock for a potential > live lock vs. tk_core.seq. Not really an improvement, right? > > The only way you can do the above is something like the incomplete and > uncompiled below. NMI safe and therefore livelock proof time interfaces > exist for a reason. Just for completeness, another approach, with its own advantages and disadvantage, is to add something like ULONG_MAX/4 to rcu_state.jiffies_stall, but also set a counter indicating that this has been done. Then RCU's force-quiescent processing could decrement that counter (if non-zero) and reset rcu_state.jiffies_stall when it does reach zero. Setting the counter to three should cover most cases, but "live by the heuristic, die by the heuristic". ;-) It would be good to have some indication when gdb exited, but things like the gdb "next" command can make that "interesting" when applied to a long-running function. Thanx, Paul > Thanks, > > tglx > --- > --- a/kernel/time/tick-sched.c > +++ b/kernel/time/tick-sched.c > @@ -51,6 +51,13 @@ struct tick_sched *tick_get_tick_sched(i > */ > static ktime_t last_jiffies_update; > > +unsigned long tick_estimate_stale_jiffies(void) > +{ > + ktime_t delta = ktime_get_mono_fast_ns() - READ_ONCE(last_jiffies_update); > + > + return delta < 0 ? 0 : div_s64(delta, TICK_NSEC); > +} > + > /* > * Must be called with interrupts disabled ! > */ > >