From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joel Fernandes <joelaf@google.com>
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you can
Date: Tue, 01 May 2018 01:18:09 +0000
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Cc: LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Tom Zanussi <tom.zanussi@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Boqun Feng <boqun.feng@gmail.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        "Cc: Frederic Weisbecker" <fweisbec@gmail.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Fenguang Wu <fengguang.wu@intel.com>,
        Baohong Liu <baohong.liu@intel.com>,
        Vedang Patel <vedang.patel@intel.com>
To: Masami Hiramatsu <mhiramat@kernel.org>
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On Wed, Apr 18, 2018 at 2:02 AM Masami Hiramatsu <mhiramat@kernel.org>
wrote:

> On Mon, 16 Apr 2018 21:07:47 -0700
> Joel Fernandes <joelaf@google.com> wrote:

> > With TRACE_IRQFLAGS, we call trace_ API too many times. We don't need
> > to if local_irq_restore or local_irq_save didn't actually do anything.
> >
> > This gives around a 4% improvement in performance when doing the
> > following command: "time find / > /dev/null"
> >
> > Also its best to avoid these calls where possible, since in this series,
> > the RCU code in tracepoint.h seems to be call these quite a bit and I'd
> > like to keep this overhead low.

> Can we assume that the "flags" has only 1 bit irq-disable flag?
> Since it skips calling raw_local_irq_restore(flags); too,
> if there is any state in the flags on any arch, it may change the
> result. In that case, we can do it as below (just skipping
trace_hardirqs_*)

> int disabled = irqs_disabled();

> if (!raw_irqs_disabled_flags(flags) && disabled)
>          trace_hardirqs_on();

> raw_local_irq_restore(flags);

> if (raw_irqs_disabled_flags(flags) && !disabled)
>          trace_hardirqs_off();

With changes to the tracepoint implementation which uses srcu instead of
rcu, I'm not able to see a performance improvement with the above patch.
For this reason, I will drop this patch from next series. thanks,

- Joel