linux-kernel - Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq offset incrementation

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Date:   Tue, 1 Dec 2020 16:01:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Tony Luck <tony.luck@...el.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH 4/5] irqtime: Move irqtime entry accounting after irq
 offset incrementation

On Tue, Dec 01, 2020 at 03:35:45PM +0100, Frederic Weisbecker wrote:
> And that one too makes things simple. But note that
> 
>     account_hardirq_enter_time()
> 
> will still need some preempt count checks to see if
> this is a nested hardirq, a hardirq interrupting a softirq
> or a hardirq interrupting a task.

So the current tests get that all correct in a single function.
Splitting it out will just result in more lines to get wrong.

That is, I don't think you can do it saner than:

  account_softirq_enter() := irqtime_account_irq(curr, SOFTIRQ_OFFSET);
  account_softirq_exit()  := irqtime_account_irq(curr, 0);
  account_hardirq_enter() := irqtime_account_irq(curr, HARDIRQ_OFFSET);
  account_hardirq_exit()  := irqtime_account_irq(curr, 0);

Fundamentally you have to determine the previous context to determine
where to account the delta to. Note that when the previous context is
task context we throw away the delta.