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

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Message-Id: <20180424182623.GA1357@linux.vnet.ibm.com>
Date:   Tue, 24 Apr 2018 11:26:23 -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 11:23:02AM -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.

							Thanx, Paul