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

From: Joel Fernandes
Date: Fri Apr 20 2018 - 03:07:26 EST


Thanks Matsami and Namhyung for the suggestions!

On Wed, Apr 18, 2018 at 10:43 PM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxx> 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?

The flags restoration I am hoping is "cheap" but I haven't measured
specifically the cost of this though.

>> 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)) {
> raw_local_irq_restore(flags);
> #endif
> return;
> }

Hmm, somehow I feel this part should be written generically enough
that it applies to all architectures (as a first step).

>> 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();

I like this idea since its a good thing to do the flag restoration
just to be safe and preserve the current behaviors. Also my goal was
to reduce the trace_ calls in this series, so its probably better I
just do as you're suggesting. I will do some experiments and make the
changes for the next series.


- Joel