On Sat, Jan 3, 2009 at 12:35 AM, Peter Teoh <htmldeveloper@xxxxxxxxx> wrote:
> On Fri, Jan 2, 2009 at 5:55 PM, Shyam Burkule <shyam.burkule@xxxxxxxxx> wrote:
>> Hi All,
>>
>> In Understanding The Linux Kernel, 3rd edition I read that
>> *scheduler_tick()* is invoked once every tick to perform some operations
>
> Yes, it is still valid and happens. Looking into /proc/sched_debug:
> CFS is confirmed.
>
> Next:
>
> Looking into kernel/timer.c:update_process_times():
>
> /*
> * Called from the timer interrupt handler to charge one tick to the current
> * process. user_tick is 1 if the tick is user time, 0 for system.
> */
> void update_process_times(int user_tick)
> {
> scheduler_tick();
>
> From above we know that timer tick triggered the function, and
> scheduler_tick() is called.
>
> Then using ftrace to analyse dynamically:
>
> <idle>-0 [000] 4154504419.814143: scheduler_tick
> <-update_process_times
> gedit-26702 [001] 4154504419.814146: scheduler_tick
> <-update_process_times
> <idle>-0 [000] 4154504419.815139: scheduler_tick
> <-update_process_times
> gedit-26702 [001] 4154504419.815142: scheduler_tick
> <-update_process_times
>
> So yes, it is still called in the context of my gedit application.
>
>> related to scheduling. I am confused here, how costly it would be to call
>> the function after every tick? Does it really happened? If not, then how
>> indication of process time slice exhausted(if any) comes to know??
>>
>>
>
> In the past, preemption is triggered by the timer. But nowadays,
> preemption is voluntary.
>
> http://lwn.net/Articles/95334/
>
> So why is it that we can still see timer based scheduling API like
> scheduler_tick() called from timer interrupt?
>
> I am confused too.
>
Sorry, I think I was wrong. The word "tickless" is more appropriate
understood as dynamic ticks. Timer do still get triggered
periodically, contrary to my previous understanding:
http://lwn.net/Articles/138969/
http://lkml.org/lkml/2008/1/14/42
And shown below through systemtap:
0xc043219d : do_timer+0x1/0xb4 [kernel]
0xc04444db : tick_sched_timer+0x48/0xa1 [kernel]
0xc043d4f5 : __run_hrtimer+0x54/0x8b [kernel]
0xc043de25 : hrtimer_interrupt+0xe8/0x147 [kernel]
0xc041463b : smp_apic_timer_interrupt+0x71/0x84 [kernel]
0xc04046cd : apic_timer_interrupt+0x2d/0x34 [kernel]
do_timer
And from below, scheduler_tick() can execute in different process
context, not just the idle as indicated in
http://lwn.net/Articles/95334/ and the patch by Scott Wood
http://lwn.net/Articles/95387/):
<idle>-0 [000] 575.341179: scheduler_tick <-update_process_times
ping-5571 [001] 575.341181: update_process_times <-tick_periodic
ping-5571 [001] 575.341183: scheduler_tick <-update_process_times
<idle>-0 [000] 575.342176: update_process_times <-tick_periodic
<idle>-0 [000] 575.342177: scheduler_tick <-update_process_times
ksoftirqd/1-7 [001] 575.342179: update_process_times <-tick_periodic
ksoftirqd/1-7 [001] 575.342181: scheduler_tick <-update_process_times
Comments?
--
Regards,
Peter Teoh
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