Messages in this thread | | | Date | Mon, 25 Sep 2006 23:39:31 +0800 | From | Aubrey <> | Subject | Re: [PATCH 1/4] Blackfin: arch patch for 2.6.18 |
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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.
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