Re: [PATCH 1/4] Blackfin: arch patch for 2.6.18

[Aubrey <aubreylee@gmail.com>]

On 9/25/06, Arnd Bergmann <arnd@arndb.de> wrote:
> It's the same problem as why sleep_on() is wrong and wait_event() is
> right, you can find lots of documentation about that.
>
> Think about a process calling nanosleep() to wait for a timeout.
> It eventually ends up going to sleep and the kernel schedules
> the idle task.
>
> The idle task checks need_resched(), which returns false, so it
> will call the idle instruction. Before it gets there, the timer
> tick happens, which calls the timer softirq. That softirq notices
> that the user process should continue running and calls wake_up(),
> which makes the condition for need_resched() true.
>
> Since we're handling that softirq that interrupted the idle task,
> that task continues what it was last doing and calls the idle instruction.
> This is the point where it goes wrong, because your user task should
> run, but the CPU is sleeping until the next interrupt happens.
>
> Note that you should in the future disable timer ticks during the
> idle function (CONFIG_NO_IDLE_HZ or similar) to save more power, but
> in that case the CPU may be in idle indefinitely after missing the
> one interrupt that should have woken it up.
>

I digged into the code and got something different.
Between need_resched() and IDLE instruction, a timer interrupt occurs.
Yes, softirq may not schedule out to run the user task, but the
interrupt handler will.
You can find in our patch, I believe the same behavior is on the ARM/M68K.

1) Timer interrupt will call do_irq(), then return_from_int().

2) return_from_int() will check if there is interrupt pending or
signal pending, if so, it will call schedule_and_signal_from_int().

3) schedule_and_signal_from_int() will jump to resume_userspace()

4) resume_userspace() will call _schedule to run the user task.

So, here is no interrupt latency. The user task will run even between
need_resched() and IDLE instruction.

-Aubrey