On Tue, Nov 20, 2018 at 02:28:14PM -0800, Paul E. McKenney wrote: > On Tue, Nov 20, 2018 at 12:42:43PM -0800, Joel Fernandes wrote: > > On Sun, Nov 11, 2018 at 10:36:18AM -0800, Paul E. McKenney wrote: > > > On Sun, Nov 11, 2018 at 10:09:16AM -0800, Joel Fernandes wrote: > > > > On Sat, Nov 10, 2018 at 08:22:10PM -0800, Paul E. McKenney wrote: > > > > > On Sat, Nov 10, 2018 at 07:09:25PM -0800, Joel Fernandes wrote: > > > > > > On Sat, Nov 10, 2018 at 03:04:36PM -0800, Paul E. McKenney wrote: > > > > > > > On Sat, Nov 10, 2018 at 01:46:59PM -0800, Joel Fernandes wrote: > > > > > > > > Hi Paul and everyone, > > > > > > > > > > > > > > > > I was tracing/studying the RCU code today in paul/dev branch > > > > > > > > and noticed that > > > > > > > > for dyntick-idle CPUs, the RCU GP thread is clearing the > > > > > > > > rnp->qsmask > > > > > > > > corresponding to the leaf node for the idle CPU, and reporting > > > > > > > > a QS on their > > > > > > > > behalf. > > > > > > > > > > > > > > > > rcu_sched-10 [003] 40.008039: rcu_fqs: > > > > > > > > rcu_sched 792 0 dti > > > > > > > > rcu_sched-10 [003] 40.008039: rcu_fqs: > > > > > > > > rcu_sched 801 2 dti > > > > > > > > rcu_sched-10 [003] 40.008041: rcu_quiescent_state_report: > > > > > > > > rcu_sched 805 5>0 0 0 3 0 > > > > > > > > > > > > > > > > That's all good but I was wondering if we can do better for the > > > > > > > > idle CPUs if > > > > > > > > we can some how not set the qsmask of the node in the first > > > > > > > > place. Then no > > > > > > > > reporting would be needed of quiescent state is needed for idle > > > > > > > > CPUs right? > > > > > > > > And we would also not need to acquire the rnp lock I think. > > > > > > > > > > > > > > > > At least for a single node tree RCU system, it seems that would > > > > > > > > avoid needing > > > > > > > > to acquire the lock without complications. Anyway let me know > > > > > > > > your thoughts > > > > > > > > and happy to discuss this at the hallways of the LPC as well > > > > > > > > for folks > > > > > > > > attending :) > > > > > > > > > > > > > > We could, but that would require consulting the rcu_data > > > > > > > structure for > > > > > > > each CPU while initializing the grace period, thus increasing the > > > > > > > number > > > > > > > of cache misses during grace-period initialization and also > > > > > > > shortly after > > > > > > > for any non-idle CPUs. This seems backwards on busy systems > > > > > > > where each > > > > > > > > > > > > When I traced, it appears to me that rcu_data structure of a remote > > > > > > CPU was > > > > > > being consulted anyway by the rcu_sched thread. So it seems like > > > > > > such cache > > > > > > miss would happen anyway whether it is during grace-period > > > > > > initialization or > > > > > > during the fqs stage? I guess I'm trying to say, the consultation > > > > > > of remote > > > > > > CPU's rcu_data happens anyway. > > > > > > > > > > Hmmm... > > > > > > > > > > The rcu_gp_init() function does access an rcu_data structure, but it > > > > > is > > > > > that of the current CPU, so shouldn't involve a communications cache > > > > > miss, > > > > > at least not in the common case. > > > > > > > > > > Or are you seeing these cross-CPU rcu_data accesses in rcu_gp_fqs() or > > > > > functions that it calls? In that case, please see below. > > > > > > > > Yes, it was rcu_implicit_dynticks_qs called from rcu_gp_fqs. > > > > > > > > > > > CPU will with high probability report its own quiescent state > > > > > > > before three > > > > > > > jiffies pass, in which case the cache misses on the rcu_data > > > > > > > structures > > > > > > > would be wasted motion. > > > > > > > > > > > > If all the CPUs are busy and reporting their QS themselves, then I > > > > > > think the > > > > > > qsmask is likely 0 so then rcu_implicit_dynticks_qs (called from > > > > > > force_qs_rnp) wouldn't be called and so there would no cache misses > > > > > > on > > > > > > rcu_data right? > > > > > > > > > > Yes, but assuming that all CPUs report their quiescent states before > > > > > the first call to rcu_gp_fqs(). One exception is when some CPU is > > > > > looping in the kernel for many milliseconds without passing through a > > > > > quiescent state. This is because for recent kernels, cond_resched() > > > > > is not a quiescent state until the grace period is something like 100 > > > > > milliseconds old. (For older kernels, cond_resched() was never an RCU > > > > > quiescent state unless it actually scheduled.) > > > > > > > > > > Why wait 100 milliseconds? Because otherwise the increase in > > > > > cond_resched() overhead shows up all too well, causing 0day test robot > > > > > to complain bitterly. Besides, I would expect that in the common > > > > > case, > > > > > CPUs would be executing usermode code. > > > > > > > > Makes sense. I was also wondering about this other thing you mentioned > > > > about > > > > waiting for 3 jiffies before reporting the idle CPU's quiescent state. > > > > Does > > > > that mean that even if a single CPU is dyntick-idle for a long period of > > > > time, then the minimum grace period duration would be atleast 3 > > > > jiffies? In > > > > our mobile embedded devices, jiffies is set to 3.33ms (HZ=300) to keep > > > > power > > > > consumption low. Not that I'm saying its an issue or anything (since > > > > IIUC if > > > > someone wants shorter grace periods, they should just use expedited > > > > GPs), but > > > > it sounds like it would be shorter GP if we just set the qsmask early > > > > on some > > > > how and we can manage the overhead of doing so. > > > > > > First, there is some autotuning of the delay based on HZ: > > > > > > #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500)) > > > > > > So at HZ=300, you should be seeing a two-jiffy delay rather than the > > > usual HZ=1000 three-jiffy delay. Of course, this means that the delay > > > is 6.67ms rather than the usual 3ms, but the theory is that lower HZ > > > rates often mean slower instruction execution and thus a desire for > > > lower RCU overhead. There is further autotuning based on number of > > > CPUs, but this does not kick in until you have 256 CPUs on your system, > > > and I bet that smartphones aren't there yet. Nevertheless, check out > > > RCU_JIFFIES_FQS_DIV for more info on this. > > > > > > But you can always override this autotuning using the following kernel > > > boot paramters: > > > > > > rcutree.jiffies_till_first_fqs > > > rcutree.jiffies_till_next_fqs > > > > Slightly related, I was just going through your patch in the dev branch > > "doc: > > Now jiffies_till_sched_qs solicits from cond_resched()". > > > > If I understand correctly, what you're trying to do is set > > rcu_data.rcu_urgent_qs if you've not heard from the CPU long enough from > > rcu_implicit_dynticks_qs. > > > > Then in the other paths, you are reading this value and similuating a > > dyntick > > idle transition even though you may not be really going into dyntick-idle. > > Actually in the scheduler-tick, you are also using it to set NEED_RESCHED > > appropriately. > > > > Did I get it right so far? > > Partially. > > The simulated dyntick-idle transition happens if the grace period extends > for even longer, so that ->rcu_need_heavy_qs gets set. Up to that point, > all that is asked for is a local-to-the-CPU report of a quiescent state.
Right, that's true. My feeling was the whole "fake a dyntick idle transition" seems to me a bit of a hack and I was thinking if not depending on that would simplify the code so we don't need the rcu_momentary_dyntick_idle. > > I was thinking if we could simplify rcu_note_context_switch (the parts that > > call rcu_momentary_dyntick_idle), if we did the following in > > rcu_implicit_dynticks_qs. > > > > Since we already call rcu_qs in rcu_note_context_switch, that would clear > > the > > rdp->cpu_no_qs flag. Then there should be no need to call > > rcu_momentary_dyntick_idle from rcu_note_context switch. > > But does this also work for the rcu_all_qs() code path? Could we not do something like this in rcu_all_qs? as some over-simplified pseudo code: rcu_all_qs() { if (!urgent_qs || !heavy_qs) return; rcu_qs(); // This clears the rdp->cpu_no_qs flags which we can monitor in // the diff in my last email (from rcu_implicit_dynticks_qs) } > > I think this would simplify cond_resched as well. Could this avoid the need > > for having an rcu_all_qs at all? Hopefully I didn't some Tasks-RCU corner > > cases.. > > There is also the code path from cond_resched() in PREEMPT=n kernels. > This needs rcu_all_qs(). Though it is quite possible that some additional > code collapsing is possible. > > > Basically for some background, I was thinking can we simplify the code that > > calls "rcu_momentary_dyntick_idle" since we already register a qs in other > > ways (like by resetting cpu_no_qs). > > One complication is that rcu_all_qs() is invoked with interrupts > and preemption enabled, while rcu_note_context_switch() is > invoked with interrupts disabled. Also, as you say, Tasks RCU. > Plus rcu_all_qs() wants to exit immediately if there is nothing to > do, while rcu_note_context_switch() must unconditionally do rcu_qs() > -- yes, it could check, but that would be redundant with the checks This immediate exit is taken care off in the above psuedo code, would that help the cond_resched performance? > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index c818e0c91a81..5aa0259c014d 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -1063,7 +1063,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data > > *rdp) > > * read-side critical section that started before the beginning > > * of the current RCU grace period. > > */ > > - if (rcu_dynticks_in_eqs_since(rdp, rdp->dynticks_snap)) { > > + if (rcu_dynticks_in_eqs_since(rdp, rdp->dynticks_snap) || > > !rdp->cpu_no_qs.b.norm) { > > If I am not too confused, this change could cause trouble for > nohz_full CPUs looping in the kernel. Such CPUs don't necessarily take > scheduler-clock interrupts, last I checked, and this could prevent the > CPU from reporting its quiescent state to core RCU. Would that still be a problem if rcu_all_qs called rcu_qs? Also the above diff is an OR condition so it is more relaxed than before. Assuming the NOHZ_FULL CPUs call cond_resched during their looping, that would trigger the rcu_all_qs -> rcu_qs path which would clear cpu_no_qs flag for that CPU right? This would result in the above diff causing a return of 1 for that CPU (from rcu_implicit_dynticks_qs). > Or am I missing something here? I think I might be the one missing something but I'm glad we are almost on the same page ;-) thanks, - Joel