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

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Date:	Tue, 21 Jun 2016 18:32:31 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Wed, 2016-06-22 at 00:28 +0200, Peter Zijlstra wrote:
> On Tue, Jun 21, 2016 at 06:23:34PM -0400, Rik van Riel wrote:
> > 
> > > > +	/*
> > > > +	 * 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.
> 
Good point. I will use __this_cpu_read here.

> > > 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 ... ;-)

No worries, I have booze with me. Everything will be
just fine! ;)

-- 
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