Fawad Lateef Wrote:
On 11/7/05, liyu <[email protected]> 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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