On Wed, Apr 25, 2018 at 02:27:08PM -0700, Joel Fernandes wrote: > On Tue, Apr 24, 2018 at 9:20 PM, Paul E. McKenney > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > [..] > >> > > >> > Sounds good, thanks. > >> > > >> > Also I found the reason for my boot issue. It was because the > >> > init_srcu_struct in the prototype was being done in an initcall. > >> > Instead if I do it in start_kernel before the tracepoint is used, it > >> > fixes it (although I don't know if this is dangerous to do like this > >> > but I can get it to boot atleast.. Let me know if this isn't the > >> > right way to do it, or if something else could go wrong) > >> > > >> > diff --git a/init/main.c b/init/main.c > >> > index 34823072ef9e..ecc88319c6da 100644 > >> > --- a/init/main.c > >> > +++ b/init/main.c > >> > @@ -631,6 +631,7 @@ asmlinkage __visible void __init start_kernel(void) > >> > WARN(!irqs_disabled(), "Interrupts were enabled early\n"); > >> > early_boot_irqs_disabled = false; > >> > > >> > + init_srcu_struct(&tracepoint_srcu); > >> > lockdep_init_early(); > >> > > >> > local_irq_enable(); > >> > -- > >> > > >> > I benchmarked it and the performance also looks quite good compared > >> > to the rcu tracepoint version. > >> > > >> > If you, Paul and other think doing the init_srcu_struct like this > >> > should be Ok, then I can try to work more on your srcu prototype and > >> > roll into my series and post them in the next RFC series (or let me > >> > know if you wanted to work your srcu stuff in a separate series..). > >> > >> That is definitely not what I was expecting, but let's see if it works > >> anyway... ;-) > >> > >> But first, I was instead expecting something like this: > >> > >> DEFINE_SRCU(tracepoint_srcu); > >> > >> With this approach, some of the initialization happens at compile time > >> and the rest happens at the first call_srcu(). > >> > >> This will work -only- if the first call_srcu() doesn't happen until after > >> workqueue_init_early() has been invoked. Which I believe must have been > >> the case in your testing, because otherwise it looks like __call_srcu() > >> would have complained bitterly. > >> > >> On the other hand, if you need to invoke call_srcu() before the call > >> to workqueue_init_early(), then you need the patch that I am beating > >> into shape. Plus you would need to use DEFINE_SRCU() and to avoid > >> invoking init_srcu_struct(). > > > > And here is the patch. I do not intend to send it upstream unless it > > actually proves necessary, and it appears that current SRCU does what > > you need. > > > > You would only need this patch if you wanted to invoke call_srcu() > > before workqueue_init_early() was called, which does not seem likely. > > Cool. So I was chatting with Paul and just to update everyone as well, > I tried the DEFINE_SRCU instead of the late init_srcu_struct call and > can make it past boot too (thanks Paul!). Also I don't see a reason we > need the RCU callback to execute early and its fine if it runs later. Very good, thank you! > Also, I was thinking of introducing a separate trace_*event*_srcu API > as a replacement to the _rcuidle API. Then I can make use of it for my > tracepoints, and then later can use it for the other tracepoints > needing _rcuidle. After that we can finally get rid of the _rcuidle > API if there are no other users of it. This is just a rough plan, but > let me know if there's any issue with this plan that you can think > off. You mean make _rcuidle use SRCU instead of RCU? Sounds reasonable to me. > IMO, I believe its simpler if the caller worries about whether it can > tolerate if tracepoint probes can block or not, than making it a > property of the tracepoint. That would also simplify the patch to > introduce srcu and keep the tracepoint creation API simple and less > confusing, but let me know if I'm missing something about this. If it helps, you can use synchronize_rcu_mult() to wait for several different types of RCU grace periods concurrently. Of course, if it is fast enough to just do a synchronize_rcu() followed by a synchronize_srcu(), why worry? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
