linux-kernel - Re: [PATCH V8 2/3] irq: Track the interrupt timings

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Date:   Thu, 23 Mar 2017 20:02:25 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        nicolas.pitre@...aro.org, rafael@...nel.org,
        vincent.guittot@...aro.org
Subject: Re: [PATCH V8 2/3] irq: Track the interrupt timings

On Thu, Mar 23, 2017 at 07:35:42PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 23, 2017 at 06:42:02PM +0100, Daniel Lezcano wrote:
> > +/*
> > + * The function record_irq_time is only called in one place in the
> > + * interrupts handler. We want this function always inline so the code
> > + * inside is embedded in the function and the static key branching
> > + * code can act at the higher level. Without the explicit
> > + * __always_inline we can end up with a function call and a small
> > + * overhead in the hotpath for nothing.
> > + */
> > +static __always_inline void record_irq_time(struct irq_desc *desc)
> > +{
> > +	if (static_key_enabled(&irq_timing_enabled)) {
> 
> I think you meant to have either static_branch_likely() or
> static_branch_unlikely() here. Those are runtime code patched,
> static_key_enabled() generates a regular load and test condition.
> 
> Also; if you do something like:
> 
> 	if (!static_branch_likely(&irq_timing_enabled))
> 		return;
> 
> you can save one level of indent.

Ok, thanks for the hint.

> > +		if (desc->istate & IRQS_TIMINGS) {
> > +			struct irq_timings *timings = this_cpu_ptr(&irq_timings);
> > +			unsigned int index = timings->count & IRQ_TIMINGS_MASK;
> > +
> > +			timings->values[index].ts = local_clock();
> > +			timings->values[index].irq = irq_desc_get_irq(desc);
> > +			timings->count++;
> > +		}
> > +	}
> > +}
> 
> 
> 
> > +DEFINE_STATIC_KEY_FALSE(irq_timing_enabled);
> > +
> > +DEFINE_PER_CPU(struct irq_timings, irq_timings);
> > +
> > +void irq_timings_enable(void)
> > +{
> > +	static_branch_inc(&irq_timing_enabled);
> 
> Do you really need counting, or do you want static_branch_enable() here?

I put counting in order to let several subsystem to use the irq timings if it
is needed.
 
> > +}
> > +
> > +void irq_timings_disable(void)
> > +{
> > +	static_branch_dec(&irq_timing_enabled);
> 
> idem.
> 
> > +}
> > -- 
> > 1.9.1
> > 

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