Re: udelay() and BogoMips doubts

[Gaurav Dhiman <gauravd_linux@xxxxxxxxx>]

 

Hi All,

 

Thanks for your replies, but nobody  answered about "BogoMips" and "loop_per_second".

If somebody knows the details of BogoMips, please let me know how it works and what is the role of it in udelay() function.

 

One more thing, as Prasanna said that while returning back from interrupt, check is doing if we are returning to kernel space or user space and then scheduler is called only if we are returning to user space and the time slot of current process has been expired. Please let me know, in case we are returning to kernel mode, why the scheduler is not called if the time slot of current process has expired, why such a decision is only taken if we are returning to user process. If the scheduler is called in this case also (when returning to kernel space), then situation like while() loop in kernel mode will not make the system hang. It will definitely bring the performance down, but will not give an impression of hanged system. Why scheduler is not called while retuning to kernel mode.

 

Regards,

Gaurav.

prasanna wakhare <prasannawakhare@xxxxxxxxx> wrote:

Hi,
Some facts about timer handler:
When Scheduler invokes in 2.4? At every timer
interrupt jiffies are incremented and check is made
if current process time slice is over or it is waiting
for I/O ,so if its need_resched flag is set the
scheduler is invoked.
After this moment,
In case of code interrupt by a timer handler, at
IRET(interrupt return) the check is made whether the
code interrupted belongs to user land or kernel mode ,
If the code belongs to user land scheduler is invoked
immediately, if code is of kernel mode the interrupted
path is resume without invoking scheduler.
Please forgive me if i was wrong B'coz i think
something i missed out.
Prasanna.


> >>- If we go into infinte loop in kernel mode, will
> the scheduler be
> called by timer interrupt once the time allocated to
> current runing process
> expires ? Does the >> timer interrupt take care of
> it or will our system be
> hanged. If timer interrupt takes care of it, how
> does it do that ?
>
> Yes, the timer will interrupt the loop [unless
> ofcourse you disable all
> interrupts prior to looping], but it will *NOT* take
> care of loop, once the
> timer interrupt service finishes, it returns to the
> loop since the loop is
> running in kernel mode and kernel is non-preemptible
> (except for
> interrupts) and the system will appear as if it is
> hanged. [No other
> processing will occur apart from interrupt
> servicing]. This is true till
> kernel 2.4. Dunno about 2.6 which has a preemptible
> kernel.
>
> Correct me if i am wrong.
>
> For BogoMips visit
> http://www.tldp.org/HOWTO/BogoMips.html
>
> Ketan
>
>
> At 10:36 PM 8/10/2004, Gaurav Dhiman wrote:
> >
> >Hi,
> >
> >Can someone let me know the answers to my following
> doubts:
> >
> >
> >
> >- I am reading the 6th chapter of Rubini's book
> (LDD), want to know about
> >the internals of udelay() function. How this
> function exactly works, I saw
> >its code at following link:
>
>http://lxr.linux.no/source/include/asm-i386/delay.h?v=2.4.21#L18
> >but cud not understand it as I am not able to find
> the definations of
> >__bad_ndelay and all other realted functions. The
> deceleration has been
> >given just before the defination of udelay()
> function, dont know where the
> >definition is.
> >Also let me know about BogoMips and how
> 'loop_per_second' is used in
> >udelay() macro.
> >
> >You can give me the links to some good articles on
> this, I can read it
> >from there.
> >
> >Regards,
> >Gaurav.
> >
> >
> >
> >Do you Yahoo!?
>
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