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SubjectRe: [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@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();

Thank you,

>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zilstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Thomas Glexiner <tglx@linutronix.de>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Fenguang Wu <fengguang.wu@intel.com>
> Cc: Baohong Liu <baohong.liu@intel.com>
> Cc: Vedang Patel <vedang.patel@intel.com>
> Signed-off-by: Joel Fernandes <joelaf@google.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@kernel.org>

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