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

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Date:   Wed, 18 Apr 2018 18:02:50 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     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>,
        Namhyung Kim <namhyung@...nel.org>,
        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>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Fenguang Wu <fengguang.wu@...el.com>,
        Baohong Liu <baohong.liu@...el.com>,
        Vedang Patel <vedang.patel@...el.com>
Subject: Re: [RFC v4 3/4] irqflags: Avoid unnecessary calls to trace_ if you
 can

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,
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();

Thank you,

> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Peter Zilstra <peterz@...radead.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Tom Zanussi <tom.zanussi@...ux.intel.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Thomas Glexiner <tglx@...utronix.de>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Fenguang Wu <fengguang.wu@...el.com>
> Cc: Baohong Liu <baohong.liu@...el.com>
> Cc: Vedang Patel <vedang.patel@...el.com>
> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
> ---
>  include/linux/irqflags.h | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
> index 9700f00bbc04..ea8df0ac57d3 100644
> --- a/include/linux/irqflags.h
> +++ b/include/linux/irqflags.h
> @@ -104,19 +104,20 @@ do {						\
>  #define local_irq_save(flags)				\
>  	do {						\
>  		raw_local_irq_save(flags);		\
> -		trace_hardirqs_off();			\
> +		if (!raw_irqs_disabled_flags(flags))	\
> +			trace_hardirqs_off();		\
>  	} while (0)
>  
>  
> -#define local_irq_restore(flags)			\
> -	do {						\
> -		if (raw_irqs_disabled_flags(flags)) {	\
> -			raw_local_irq_restore(flags);	\
> -			trace_hardirqs_off();		\
> -		} else {				\
> -			trace_hardirqs_on();		\
> -			raw_local_irq_restore(flags);	\
> -		}					\
> +#define local_irq_restore(flags)						\
> +	do {									\
> +		if (raw_irqs_disabled_flags(flags) && !irqs_disabled()) {	\
> +			raw_local_irq_restore(flags);				\
> +			trace_hardirqs_off();					\
> +		} else if (!raw_irqs_disabled_flags(flags) && irqs_disabled()) {\
> +			trace_hardirqs_on();					\
> +			raw_local_irq_restore(flags);				\
> +		}								\
>  	} while (0)
>  
>  #define safe_halt()				\
> -- 
> 2.17.0.484.g0c8726318c-goog
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>