Messages in this thread | | | Date | Wed, 22 Jun 2016 00:28:32 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 5/5] irqtime: drop local_irq_save/restore from irqtime_account_irq |
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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 ... ;-)
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