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Message-ID: <1e62d1370511071853o5d65f0dcm7548b7563615ed35@mail.gmail.com>
Date: Tue, 8 Nov 2005 07:53:34 +0500
From: Fawad Lateef <fawadlateef@gmail.com>
To: liyu <liyu@ccoss.com.cn>
Subject: Re: [question] I doublt on timer interrput.
Cc: rml@novell.com, LKML <linux-kernel@vger.kernel.org>
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On 11/8/05, liyu <liyu@ccoss.com.cn> wrote:
> Fawad Lateef Wrote:
>
> >
> >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 !
> >
> 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.
>

Yes, this is the thing which I missed


> 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.
>

By saying __might_be_called_from_somewhere__ I meant that I am missing
some-other place __with-in_the_kernel_code__ from where it is called,
which you pointed to me (about do_timer.h)   :)

> 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.
>

If you are declaring count_tick as a global variable (without static)
in sched.c then you can just use it in your test module by specifying
extern for your variable

>
> 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 ?!
>

Yes, I am now sure that it will be called for every timer interrupt ! :)

Thanks,

--
Fawad Lateef
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