Re: [question] I doublt on timer interrput.

[liyu]

Fawad Lateef Wrote:

> On 11/7/05, liyu <liyu@xxxxxxxxxxxx> wrote:
>  
>
> >    I have one question about timer interrupt (i386 architecture).
> >
> >    As we known, the timer emit HZ times interrputs per second,
> > and in i386. The interrupt handler will call scheduler_tick()
> > each time (on i386 at least, both enable or disable APIC).
> >
> >    On my Celeron machine(IOW, only one CPU, not SMP/SMT), I defined
> > a global int variable 'tick_count' in kernel/sched.c, and add one line
> > of code like follow in scheduler_tick():
> >
> >    ++tick_count;
> >
> >    but I found it is not same with content of the /proc/interrupts,
> > and the differennt between them is not little.
> >
> >    I can not understand why that is.
> >
> >    Any useful idea.
> >
> >
> >    
>
> What I found in the kernel code is that scheduler_tick is called from
> two locations in the kernel (2.6.14-mm1) code (i386).
>
> 1) from kernel/timer.c in update_process_times which is called from
> arch/i386/kernel/apic.c and its calling depends on the CONFIG_SMP
> defined or not (see
> http://sosdg.org/~coywolf/lxr/source/arch/i386/kernel/apic.c#L1160)
> and as you don't have CONFIG_SMP enabled so its won't be called from
> here.
>
> 2) from sched_fork function in kernel/sched.c
> (http://sosdg.org/~coywolf/lxr/source/kernel/sched.c#L1414) and I
> think its called when newly forked process setup is going to be
> performed, and I think as from here scheduler_tick is called in your
> case, so you are getting different value for your variable tick_count
>
> scheduler_tick might be called from somewhere else which I am missing
> so please CMIIW !
>
> --
> Fawad Lateef
> -
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>
>  
Please see this URL:

http://lxr.linux.no/source/include/asm-i386/mach-default/do_timer.h#L20

static inline void do_timer_interrupt_hook(struct pt_regs *regs)
{
       do_timer(regs);
#ifndef CONFIG_SMP
       update_process_times(user_mode(regs));
#endif
/*
* In the SMP case we use the local APIC timer interrupt to do the
* profiling, except when we simulate SMP mode on a uniprocessor
* system, in that case we have to call the local interrupt handler.
*/
#ifndef CONFIG_X86_LOCAL_APIC
       profile_tick(CPU_PROFILING, regs);
#else
       if (!using_apic_timer)
               smp_local_timer_interrupt(regs);
#endif
}


That is the code in 2.6.12, but 2.6.13.3 also same with it at least.
So we call scheduler_tick() HZ times per second, both enable
SMP or disable it.

Nod, I agree with your words, the scheduler_tick() do not same with
timer interrupt handler on call times. but I guess it should be more
than jiffies, beacause of other functions also can call it (for example,
as Lateef said, sched_fork().)

I think that

scheduler_tick() might be called from somewhere

is not exact.

We may note, it do not be EXPORT_SYMBOL_*()ed , so it only can be called 
from kernel core,
not kernel modules. Such a few places we can find it use LXR or grep.

I use setup one sysctl integer variable to watch the value of 'count_tick',
Do this way have any problem? I found some value skips, but I think it is
normal case.


However, I will make a experiemnt that write one hook like do_timer(), 
as Love said

PS: if our scheduler_tick() is not called every timer interrput, the 
compute of task timeslice
also is not exact ?!

Thanks a lot.


-liyu

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