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

From: Joel Fernandes <joelaf@google.com>
To: Paul McKenney <paulmck@linux.vnet.ibm.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 09:01:34 -0700	[thread overview]
Message-ID: <CAJWu+or_-emRLeBbfOtQYyn9W8v-KHH8XecV_k-p5FX_3BLHAQ@mail.gmail.com> (raw)
In-Reply-To: <20180424155655.GA820@linux.vnet.ibm.com>

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.

thanks,

- Joel