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

[Thomas Gleixner]

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.