On Thu, Oct 30, 2014 at 05:54:35PM +0000, Alexander Gordeev wrote: > On Wed, Oct 29, 2014 at 05:05:24PM -0700, Paul E. McKenney wrote: > > > > Although it is true that tiny RCU cannot hang a synchronize_rcu() > > > > grace period, it most certainly can hang a call_rcu() grace period > > > > in exactly the same way. > > > > > > Sorry for being a pain in the neck - just want to make sure I am > > > following. > > > > No worries! > > > > > I only see possibility to cause callbacks not being called for "too long" > > > in case a system has lots of nested interrupts and rcu_idle_enter_common() > > > is not being called from hardware interrupt context as result. How could > > > rcu_is_cpu_rrupt_from_idle() help here? > > > > Let's start assuming that something in the idle loop posts a callback, > > and then let me see if I understand your reasoning... > > > > 1. The system is idle and stays that way, no runnable tasks. > > > > 2. An interrupt occurs. Upon return from interrupt, rcu_irq_exit() > > is invoked, which calls rcu_idle_enter_common(), which in turn > > calls rcu_sched_qs(), which does a raise_softirq(RCU_SOFTIRQ). > > > > 3. The softirq happens shortly and invokes rcu_process_callbacks(), > > which invokes __rcu_process_callbacks(). > > > > 4. So now callbacks can be invoked. At least they can be if > > ->donetail has been updated. Which it will have been because > > rcu_sched_qs() invokes rcu_qsctr_help(). > > Yes, that is exactly my reasoning. > > > So your point that rcu_is_cpu_rrupt_from_idle() might be redundant could > > well be valid -- sorry for being so dismissive earlier. > > > > > > > > Now, if you -can- get the userspace-execution indication into > > > > > > rcu_irq_exit(), this might be of interest. However, it might be > > > > > > faster > > > > > > to simply let the scheduling-clock interrupt do the job as it > > > > > > currently > > > > > > does, especially for workloads with lots of interrupts. > > > > > > > > > > > > Or did you have something else in mind? > > > > > > > > > > Nope. I would even leave as is tiny RCU's rcu_is_cpu_rrupt_from_idle() > > > > > for clarity then ;) > > > > > > > > Also to avoid userspace execution from preventing RCU callbacks from > > > > ever being invoked. ;-) > > > > > > Hmm.. Am I missing something else? I did not remove the userspace check > > > from the scheduling-clock interrupt: > > > > > > @@ -250,7 +240,7 @@ void rcu_bh_qs(void) > > > void rcu_check_callbacks(int cpu, int user) > > > { > > > RCU_TRACE(check_cpu_stalls()); > > > - if (user || rcu_is_cpu_rrupt_from_idle()) > > > + if (user) > > > rcu_sched_qs(); > > > else if (!in_softirq()) > > > rcu_bh_qs(); > > > > Probably just me being confused. Hopefully so, as shrinking TINY_RCU > > further will probably be welcome. > > Should I resend tiny-only patch?
Please, but also expand the commit log to contain the above reasoning. > > Have you done any testing of this change? > > Just booted to a unicore kernel and dd'ed 1G of /dev/sda to /dev/null. OK, that is a start. Could I convince you to build a kernel? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/