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Re: tasklet latency and system calls on mips

To: Jun Sun <jsun@mvista.com>
Subject: Re: tasklet latency and system calls on mips
From: "Sirotkin, Alexander" <demiurg@ti.com>
Date: Thu, 14 Aug 2003 12:17:52 +0300
Cc: linux-mips@linux-mips.org
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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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