Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can

From: Paul E. McKenney
Date: Tue Apr 24 2018 - 20:09:55 EST

On Tue, Apr 24, 2018 at 04:46:54PM -0700, Joel Fernandes wrote:
> On 04/24/2018 04:21 PM, Mathieu Desnoyers wrote:
> >----- On Apr 24, 2018, at 2:59 PM, Joel Fernandes joelaf@xxxxxxxxxx wrote:
> >>On Tue, Apr 24, 2018 at 11:26 AM, Paul E. McKenney
> >><paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>>On Tue, Apr 24, 2018 at 11:23:02AM -0700, Paul E. McKenney wrote:
> >>>>On Tue, Apr 24, 2018 at 10:26:58AM -0700, Paul E. McKenney wrote:
> >>>>>On Tue, Apr 24, 2018 at 09:01:34AM -0700, Joel Fernandes wrote:
> >>>>>>On Tue, Apr 24, 2018 at 8:56 AM, Paul E. McKenney
> >>>>>><paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> >>>>>>>On Mon, Apr 23, 2018 at 05:22:44PM -0400, Steven Rostedt wrote:
> >>>>>>>>On Mon, 23 Apr 2018 13:12:21 -0400 (EDT)
> >>>>>>>>Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>>I'm inclined to explicitly declare the tracepoints with their given
> >>>>>>>>>synchronization method. Tracepoint probe callback functions for currently
> >>>>>>>>>existing tracepoints expect to have preemption disabled when invoked.
> >>>>>>>>>This assumption will not be true anymore for srcu-tracepoints.
> >>>>>>>>
> >>>>>>>>Actually, why not have a flag attached to the tracepoint_func that
> >>>>>>>>states if it expects preemption to be enabled or not? If a
> >>>>>>>>trace_##event##_srcu() is called, then simply disable preemption before
> >>>>>>>>calling the callbacks for it. That way if a callback is fine for use
> >>>>>>>>with srcu, then it would require calling
> >>>>>>>>
> >>>>>>>> register_trace_##event##_may_sleep();
> >>>>>>>>
> >>>>>>>>Then if someone uses this on a tracepoint where preemption is disabled,
> >>>>>>>>we simply do not call it.
> >>>>>>>
> >>>>>>>One more stupid question... If we are having to trace so much stuff
> >>>>>>>in the idle loop, are we perhaps grossly overstating the extent of that
> >>>>>>>"idle" loop? For being called "idle", this code seems quite busy!
> >>>>>>
> >>>>>>;-)
> >>>>>>The performance hit I am observing is when running a heavy workload,
> >>>>>>like hackbench or something like that. That's what I am trying to
> >>>>>>correct.
> >>>>>>By the way is there any limitation on using SRCU too early during
> >>>>>>boot? I backported Mathieu's srcu tracepoint patches but the kernel
> >>>>>>hangs pretty early in the boot. I register lockdep probes in
> >>>>>>start_kernel. I am hoping that's not why.
> >>>>>>
> >>>>>>I could also have just screwed up the backporting... may be for my
> >>>>>>testing, I will just replace the rcu API with the srcu instead of all
> >>>>>>of Mathieu's new TRACE_EVENT macros for SRCU, since all I am trying to
> >>>>>>do right now is measure the performance of my patches with SRCU.
> >>>>>
> >>>>>Gah, yes, there is an entry on my capacious todo list on making SRCU
> >>>>>grace periods work during early boot and mid-boot. Let me see what
> >>>>>I can do...
> >>>>
> >>>>OK, just need to verify that you are OK with call_srcu()'s callbacks
> >>>>not being invoked until sometime during core_initcall() time. (If you
> >>>>really do need them to be invoked before that, in theory it is possible,
> >>>>but in practice it is weird, even for RCU.)
> >>>
> >>>Oh, and that early at boot, you will need to use DEFINE_SRCU() or
> >>>DEFINE_STATIC_SRCU() rather than dynamic allocation and initialization.
> >>>
> >>> Thanx, Paul
> >>>
> >>
> >>Oh ok.
> >>
> >>About call_rcu, calling it later may be an issue since we register the
> >>probes in start_kernel, for the first probe call_rcu will be sched,
> >>but for the second one I think it'll try to call_rcu to get rid of the
> >>first one.
> >>
> >>This is the relevant code that gets called when probes are added:
> >>
> >>static inline void release_probes(struct tracepoint_func *old)
> >>{
> >> if (old) {
> >> struct tp_probes *tp_probes = container_of(old,
> >> struct tp_probes, probes[0]);
> >> call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> >> }
> >>}
> >>
> >>Maybe we can somehow defer the call_srcu until later? Would that be possible?
> >>
> >>also Mathieu, you didn't modify the call_rcu_sched in your prototype
> >>to be changed to use call_srcu, should you be doing that?
> >
> >You're right, I think I should have introduced a call_srcu in there.
> >It's missing in my prototype.
> >
> >However, in the prototype I did, we need to wait for *both* sched-rcu
> >and SRCU grace periods, because we don't track which site is using which
> >rcu flavor.
> >
> >So you could achieve this relatively easily by means of two chained
> >RCU callbacks, e.g.:
> >
> >release_probes() calls call_rcu_sched(... , rcu_free_old_probes)
> >
> >and then in rcu_free_old_probes() do:
> >
> >call_srcu(... , srcu_free_old_probes)
> >
> >and perform kfree(container_of(head, struct tp_probes, rcu));
> >within srcu_free_old_probes.
> >
> >It is somewhat a hack, but should work.
> 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:


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().

Thanx, Paul