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

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Date:   Thu, 19 Apr 2018 14:43:02 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Joel Fernandes <joelaf@...gle.com>, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        Thomas Glexiner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Fenguang Wu <fengguang.wu@...el.com>,
        Baohong Liu <baohong.liu@...el.com>,
        Vedang Patel <vedang.patel@...el.com>, kernel-team@....com
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you
 can

On Wed, Apr 18, 2018 at 06:02:50PM +0900, Masami Hiramatsu wrote:
> On Mon, 16 Apr 2018 21:07:47 -0700
> Joel Fernandes <joelaf@...gle.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,

I don't know how many it impacts on performance but maybe we can have
an arch-specific config option something like below?


> 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 (disabled == raw_irqs_disabled_flags(flags)) {
#ifndef CONFIG_ARCH_CAN_SKIP_NESTED_IRQ_RESTORE
        raw_local_irq_restore(flags);
#endif
        return;
  }

> 
> 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();

Thanks,
Namhyung