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

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Date:   Tue, 01 Dec 2020 16:53:02 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     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 16:01, Peter Zijlstra wrote:
> 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.

Fair enough.