linux-kernel - Re: [PATCH 5/5] irqtime: drop local_irq_save/restore from irqtime_account

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Message-ID: <20160621214934.GT30909@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jun 2016 23:49:34 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	pbonzini@...hat.com, fweisbec@...il.com, wanpeng.li@...mail.com,
	efault@....de, tglx@...utronix.de, rkrcmar@...hat.com
Subject: Re: [PATCH 5/5] irqtime: drop local_irq_save/restore from
 irqtime_account_irq

On Thu, Jun 16, 2016 at 12:06:07PM -0400, riel@...hat.com wrote:
> @@ -53,36 +56,72 @@ DEFINE_PER_CPU(seqcount_t, irq_time_seq);
>   * softirq -> hardirq, hardirq -> softirq
>   *
>   * When exiting hardirq or softirq time, account the elapsed time.
> + *
> + * When exiting softirq time, subtract the amount of hardirq time that
> + * interrupted this softirq run, to avoid double accounting of that time.
>   */
>  void irqtime_account_irq(struct task_struct *curr, int irqtype)
>  {
> +	u64 prev_softirq_start;
> +	u64 prev_hardirq;
> +	u64 hardirq_time;
> +	s64 delta = 0;

We appear to always assign to delta, so this initialization seems
superfluous.

>  	int cpu;
>  
>  	if (!sched_clock_irqtime)
>  		return;
>  
>  	cpu = smp_processor_id();

Per this smp_processor_id() usage, preemption is disabled.

> +	/*
> +	 * Softirq context may get interrupted by hardirq context,
> +	 * on the same CPU. At softirq entry time the amount of time
> +	 * spent in hardirq context is stored. At softirq exit time,
> +	 * the time spent in hardirq context during the softirq is
> +	 * subtracted.
> +	 */
> +	prev_hardirq = __this_cpu_read(prev_hardirq_time);
> +	prev_softirq_start = __this_cpu_read(softirq_start_time);
> +
> +	if (irqtype == HARDIRQ_OFFSET) {
> +		delta = sched_clock_cpu(cpu) - __this_cpu_read(hardirq_start_time);
> +		__this_cpu_add(hardirq_start_time, delta);
> +	} else do {
> +		u64 now = sched_clock_cpu(cpu);
> +		hardirq_time = READ_ONCE(per_cpu(cpu_hardirq_time, cpu));

Which makes this per_cpu(,cpu) usage somewhat curious. What's wrong with
__this_cpu_read() ?

> +
> +		delta = now - prev_softirq_start;
> +		if (in_serving_softirq()) {
> +			/*
> +			 * Leaving softirq context. Avoid double counting by
> +			 * subtracting hardirq time from this interval.
> +			 */
> +			s64 hi_delta = hardirq_time - prev_hardirq;
> +			delta -= hi_delta;
> +		} else {
> +			/* Entering softirq context. Note start times. */
> +			__this_cpu_write(softirq_start_time, now);
> +			__this_cpu_write(prev_hardirq_time, hardirq_time);
> +		}
> +		/*
> +		 * If a hardirq happened during this calculation, it may not
> +		 * have gotten a consistent snapshot. Try again.
> +		 */
> +	} while (hardirq_time != READ_ONCE(per_cpu(cpu_hardirq_time, cpu)));

That whole thing is somewhat hard to read; but its far too late for me
to suggest anything more readable :/

> +	irq_time_write_begin(irqtype);
>  	/*
>  	 * We do not account for softirq time from ksoftirqd here.
>  	 * We want to continue accounting softirq time to ksoftirqd thread
>  	 * in that case, so as not to confuse scheduler with a special task
>  	 * that do not consume any time, but still wants to run.
>  	 */
> +	if (irqtype == HARDIRQ_OFFSET && hardirq_count())
>  		__this_cpu_add(cpu_hardirq_time, delta);
> +	else if (irqtype == SOFTIRQ_OFFSET && in_serving_softirq() &&
> +				curr != this_cpu_ksoftirqd())
>  		__this_cpu_add(cpu_softirq_time, delta);
>  
> +	irq_time_write_end(irqtype);

Maybe split the whole thing on irqtype at the very start, instead of the
endless repeated branches?

>  }
>  EXPORT_SYMBOL_GPL(irqtime_account_irq);