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

[Joel Fernandes]

On Tue, Apr 24, 2018 at 12:09 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Apr 24, 2018 at 11:59:32AM -0700, Joel Fernandes wrote:
>> Hi Paul,
>>
>> 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.
>>
>> 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?
>
> You will be able to invoke call_srcu() early if you wish, it is just that
> the specified SRCU callback won't be invoked until core_initcall() time.
>

Yes, that should be fine then.

Also I think I see no issue with static initialization and allocation
as you were suggesting in your earlier email so that's also Ok.

Let me know when you have anything I can test, thanks a lot. I'll
pause my testing of srcu for now then and focus on the other parts of
my patchset.

thanks,

- Joel