Re: tasklet latency and system calls on mips

["Sirotkin, Alexander" <demiurg@ti.com>]

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Hi.

See my comments below.

Jun Sun wrote:


On Wed, Aug 13, 2003 at 04:46:04PM +0300, Sirotkin, Alexander wrote:

  

Hello dearest all.



I have a question regarding tasklets on MIPS. I suspect that there is a

bug in generic MIPS kernel, but I'm not sure yet.



Linux kernel has a couple of so called "checkpoints" when the system

should check if there are tasklets to

run and run them in the following way :



if (softirq_pending(cpu))

                    do_softirq();



One of these places is at the end of interrupt handler (do_IRQ()),

however this is not the only place. I was under

impression that this code should be called after system call too. The

caveat here is that on MIPS (contrary to

other architectures, such as x86) system call is not an interrupt (it's

a different exception) and has completely

different handler. So in x86 it is sufficient to call



if (softirq_pending(cpu))

                    do_softirq();



at the end of do_IRQ because do_IRQ handles system call too, but on MIPS

it is not. Therefore I believe

these lines should be added to the end of sys_syscall function on MIPS.



What do you think ?



    



softirq/tasklet/bottom_half/etc should only be raised from interrupt

context.  Checking at the end of do_IRQ should be good enough.

  

On mips interrupt is an exception and system call is a different
exception. Different exceptions has different exception handlers,
at least that's what I was able to figure from entry.S file. So the
system call does not go through do_IRQ and do_softirq
is not called.





One possible mistake in MIPS porting is that if the board uses its
private

time interrupt routine poeple may forget to put the above two lines

at the end.  Check against that.

  

In our kernel port we do have these lines in the timer interrupt, that
is not a problem. 



P.S. The whole issue started when we noticed that user process making

many system calls has very

significant impact on device drivers running in tasklet mode 

    



What kind of impact?  On i386? Or on MIPS?



  

Jun


The impact is that if our driver works in tasklet mode then some user
mode application making system calls
causes some (although quite small) packet loss. It does not happen if we
don't use tasklet and do everything in the
ISR.

I suspect that what happens is as follows :

system call arrives and while it's being processed and interrupt to one
of the drivers arrives. This interrupt 
schedules a tasklet which however is not executed after the system call
finishes, only after the next timer
interrupt which causes up to 10 ms latency (not all the time, only when
somebody makes a system call).

It only happens on MIPS. There is no easy way to check this on x86.
-- 

Alexander Sirotkin

SW Engineer



Texas Instruments

Broadband Communications Israel (BCIL)

Tel:  +972-9-9706587

________________________________________________________________________

"Those who do not understand Unix are condemned to reinvent it, poorly."

      -- Henry Spencer 

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