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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Joel Fernandes <joelaf@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Tom Zanussi <tom.zanussi@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Boqun Feng <boqun.feng@gmail.com>, fweisbec <fweisbec@gmail.com>,
	Randy Dunlap <rdunlap@infradead.org>,
	kbuild test robot <fengguang.wu@intel.com>,
	baohong liu <baohong.liu@intel.com>,
	vedang patel <vedang.patel@intel.com>,
	kernel-team <kernel-team@lge.com>
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can
Date: Tue, 24 Apr 2018 12:09:05 -0700	[thread overview]
Message-ID: <20180424190905.GU26088@linux.vnet.ibm.com> (raw)
In-Reply-To: <CAJWu+opC9pLxoiXq7QDRUDe6ouC4ij398bHCfnUwp+h7Y_ssDg@mail.gmail.com>

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@linux.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@linux.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@efficios.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
> 

  parent reply	other threads:[~2018-04-24 19:08 UTC|newest]

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-17  4:07 [RFC v4 0/4] Centralize and unify usage of preempt/irq tracepoints Joel Fernandes
2018-04-17  4:07 ` [RFC v4 1/4] tracepoint: Add API to not do lockdep checks during RCU ops Joel Fernandes
2018-04-17  4:07 ` [RFC v4 2/4] softirq: reorder trace_softirqs_on to prevent lockdep splat Joel Fernandes
2018-04-17  4:07 ` [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can Joel Fernandes
2018-04-18  9:02   ` Masami Hiramatsu
2018-04-19  5:43     ` Namhyung Kim
2018-04-20  7:07       ` Joel Fernandes
2018-04-23  1:14         ` Joel Fernandes
2018-04-23  3:19           ` Paul E. McKenney
2018-04-23 14:31             ` Mathieu Desnoyers
2018-04-23 14:53               ` Steven Rostedt
2018-04-23 14:59                 ` Mathieu Desnoyers
2018-04-23 15:12                   ` Paul E. McKenney
2018-04-23 16:18                   ` Steven Rostedt
2018-04-23 17:12                     ` Mathieu Desnoyers
2018-04-23 17:24                       ` Joel Fernandes
2018-04-23 21:22                       ` Steven Rostedt
2018-04-24 15:56                         ` Paul E. McKenney
2018-04-24 16:01                           ` Joel Fernandes
2018-04-24 17:26                             ` Paul E. McKenney
2018-04-24 18:23                               ` Paul E. McKenney
2018-04-24 18:26                                 ` Paul E. McKenney
2018-04-24 18:59                                   ` Joel Fernandes
2018-04-24 19:01                                     ` Joel Fernandes
2018-04-24 19:09                                     ` Paul E. McKenney [this message]
2018-04-24 19:16                                       ` Joel Fernandes
2018-04-24 23:21                                     ` Mathieu Desnoyers
2018-04-24 23:46                                       ` Joel Fernandes
2018-04-25  0:10                                         ` Paul E. McKenney
2018-04-25  4:20                                           ` Paul E. McKenney
2018-04-25 21:27                                             ` Joel Fernandes
2018-04-25 21:35                                               ` Paul E. McKenney
2018-04-25 21:40                                               ` Mathieu Desnoyers
2018-04-25 22:51                                                 ` Steven Rostedt
2018-04-26 15:03                                                   ` Mathieu Desnoyers
2018-04-26 16:08                                                     ` Mathieu Desnoyers
2018-04-25 23:13                                                 ` Joel Fernandes
2018-04-26 15:13                                                   ` Mathieu Desnoyers
2018-04-26 15:20                                                     ` Joel Fernandes
2018-04-26 15:49                                                     ` Paul E. McKenney
2018-04-23 15:49                 ` Joel Fernandes
2018-04-26  2:18             ` Joel Fernandes
2018-05-01  1:18     ` Joel Fernandes
2018-04-17  4:07 ` [RFC v4 4/4] tracing: Centralize preemptirq tracepoints and unify their usage Joel Fernandes

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