linux-kernel - Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace

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Date:   Tue, 24 Apr 2018 12:09:05 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        fweisbec <fweisbec@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        kbuild test robot <fengguang.wu@...el.com>,
        baohong liu <baohong.liu@...el.com>,
        vedang patel <vedang.patel@...el.com>,
        kernel-team <kernel-team@....com>
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you
 can

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@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com> 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@...icios.com> 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.

							Thanx, Paul

> 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
>