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

[Joel Fernandes]

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.
>
>                                                         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?

thanks,

 - Joel