Re: Interrupt Bottom Half Scheduling

[Sven-Thorsten Dietrich <thebigcorporation@gmail.com>]

On Mon, 2011-02-14 at 15:08 -0800, Peter LaDow wrote:
> Note sure how that is possible.  This is related to my earlier posting
> about timing jitter.  Our code is basically this:
> 

Where is the semicolon ending the for loop?


> while(1)
> {
>   t1 = clock_gettime()
>   for(i=0; i < 10000; i++)
>   t2 = clock_gettime()
> 
>   diff = t2 - 1
> }
> 
> This task is pending on nothing.  Unless clock_gettime() causes some
> sort of priority inversion, I don't see the problem.  We see
> significant jitter on the for-loop when there are a significant number
> of other kernel timers.  Now, as we understand it, the hrtimers run in
> the softirq.  But if the softirq is priority 50, and this for-loop is
> priority 99, it shouldn't be affected by the softirq thread.
> 
> Pete
> 
> On Mon, Feb 14, 2011 at 3:04 PM, Sven-Thorsten Dietrich
> <thebigcorporation@gmail.com> wrote:
> > On Mon, 2011-02-14 at 14:31 -0800, Peter LaDow wrote:
> >> How is the scheduling of the hrtimers softirq thread handled?
> >>
> >> When querying the RT priority of the hrtimer softirq, I get a priority
> >> of 50.  But when running a priority 99 thread, we still seem to be
> >> getting interrupted.  Shouldn't the hrtimer softirq be put off until
> >> the CPU is idle or a lower priority task is running?
> >
> > Does your prio 99 thread perhaps encounter a prio inversion dependency
> > on one of the softriq threads?
> >
> >>
> >> Thanks,
> >> Pete
> >> --
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> >
> >