[question] I doublt on timer interrput.

[question] I doublt on timer interrput.

[liyu]

Hi, all:

    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.



-liyu / NOW~




   

Re: [question] I doublt on timer interrput.

[Fawad Lateef]

On 11/7/05, liyu <liyu@ccoss.com.cn> 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

Re: [question] I doublt on timer interrput.

[liyu]

Fawad Lateef Wrote:

>On 11/7/05, liyu <liyu@ccoss.com.cn> 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
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
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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

Re: [question] I doublt on timer interrput.

[Fawad Lateef]

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

Re: [question] I doublt on timer interrput.

[liyu]

I get it !

I am sorry to spend your time so much, all trouble are came from one my 
low-level fault.

I put "++count_tick" in scheduler_tick(), but that function() can return 
before it!

This is one sample result:

> [root@CCOSS_629884359 root]# cat /proc/sys/kernel/count_tick 
> /proc/interrupts
> 157491
>            CPU0
>   0:     157390    IO-APIC-edge  timer
>   1:         10    IO-APIC-edge  i8042
>   8:          1    IO-APIC-edge  rtc
>   9:          0   IO-APIC-level  acpi
>  12:        111    IO-APIC-edge  i8042
>  14:      57616    IO-APIC-edge  ide0
>  15:          2    IO-APIC-edge  ide1
>  16:          0   IO-APIC-level  uhci_hcd:usb1
>  17:          0   IO-APIC-level  uhci_hcd:usb2
>  18:        893   IO-APIC-level  eth0
> NMI:          0
> LOC:     157326
> ERR:          0


We can see:

157491 > 157390 !

Yeah, I got it.

Thanks a lot again.

-liyu

Fawad Lateef wrote:

>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
>
>
>  
>

Re: [question] I doublt on timer interrput.

[Robert Love]

On Mon, 2005-11-07 at 14:05 +0800, liyu wrote:
> Hi, all:
> 
>     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.

scheduler_tick() is not the timer interrupt.

You want to hook into do_timer() or similar.

	Robert Love