On Thu, Aug 24, 2023 at 04:40:42AM -0700, Paul E. McKenney wrote: > On Thu, Aug 24, 2023 at 10:50:41AM +0800, Huacai Chen wrote: > > Hi, Paul, > > > > On Thu, Aug 24, 2023 at 6:41 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > > > > > 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. > > > > The original code is adding ULONG_MAX/2, so adding ULONG_MAX/4 may > > make no much difference? The simplest way is adding 300*HZ, but Joel > > dislikes that. > > I am not seeing the ULONG_MAX/2, so could you please point me to that > original code? > > The advantage of ULONG_MAX/4 over ULONG_MAX/2 is that the time_after() > and time_before() macros have ULONG_MAX/4 slop in either direction > before giving you the wrong answer. You can get nearly the same result > using ULONG_MAX/2, but it requires a bit more care. And even on 32-bit > HZ=1000 systems, ULONG_MAX/4 gets you more than 12 days of gdb session > or jiffies-update delay before you start getting false positives. > > Then things can be reset after (say) 3 calls to rcu_gp_fqs() and > also the current reset at the beginning of a grace period, which > is in record_gp_stall_check_time(). I like Paul's suggestion a lot except that if someone sets a breakpoint right when the jiffies is being reset, so then we have to come back to doing Thomas's suggestion. So maybe a combination of Paul's and Thomas's suggestions (of using last_jiffies_update with the NMI-safe timestamp read) may work. > It would be better if RCU could get notified at both ends of the debug > session, but given gdb commands such as "next", along with Thomas's > point about gdb breakpoints being pretty much anywhere, this might or > might not be so helpful in real life. But worth looking into. True, I was curious if rcu_cpu_stall_reset() can be called on a tickless kernel as well before jiffies gets a chance to update, in which case I think your suggestion of biasing the stall time and later resetting it would help a lot for such situations. thanks, - Joel > Thanx, Paul > > > Huacai > > > > > > > > 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 ! > > > > */ > > > > > > > >