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

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Message-ID: <20160621222832.GU30909@twins.programming.kicks-ass.net>
Date:	Wed, 22 Jun 2016 00:28:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <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 Tue, Jun 21, 2016 at 06:23:34PM -0400, Rik van Riel wrote:
> > >  	cpu = smp_processor_id();
> > Per this smp_processor_id() usage, preemption is disabled.
> 
> This code is called from the timer code. Surely preemption
> is already disabled?

That's what I said.

> > > 
> > > +	/*
> > > +	 * 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() ?
> 
> Is __this_cpu_read() as fast as per_cpu(,cpu) on all
> architectures?

Can't be slower. Don't get the argument though; you've used __this_cpu
stuff all over the place, and here you use a per_cpu() for no reason.

> > > 
> > > +
> > > +		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 :/
> 
> I only had 2 1/2 hours of sleep last night, so I will not
> try to rewrite it now, but I will see if there is anything
> I can do to make it more readable tomorrow.
> 
> If you have any ideas before then, please let me know :)

Heh, step away from the computer ... ;-)