> I've written a small test program which enables periodic RTC interrupts
> at 8192 Hz and then goes into a loop reading /dev/rtc and collecting
> timing statistics (using the rdtscl macro).
straightforward test, used for many years in the linux community
(I claim to have been the first to publish it on lkml ;)
> The test system runs a 2.6.11 kernel (no SMP) on a Pentium3 500 MHz
> embedded hardware.
which probably has memory bandwidth of at most a couple hundred MB/s,
which is really horrible by modern standards.
> However, things break seriously when exercising the CF card in parallel
> (e.g. with a dd if=/dev/hda of=/dev/null):
>
> * The rtc *interrupt handler* is delayed for up to 250 *micro*seconds.
> This is very bad for my purpose, but easy to explain: It is roughly the
> time needed to transfer 512 Bytes from a CF card which can transfer 2
> Mbyte/sec, and obviously, the CPU blocks all interrupts while making pio
>
> transfers. (Why? Is this really necessary?)
even with -u1, isn't there still a softirq queue that will delay the wakeup
of your user-space tester?
> * The *test program* is regularly blocked for 63 *milli*seconds,
> sometimes for up to 300 *milli*seconds, which is absolutely
> unacceptable.
guessing that's VM housekeeping.
> Now the big question:
> *** Why doesn't my rt test program get *any* CPU for 63 jiffies? ***
> (the system ticks at 1000 HZ)
because it's user-space. the 'rt' is a bit of a misnomer - it's merely
a higher priority, less preemptable job.
> * The dd program obviously gets some CPU regularly (because it copies 2
> MB/s, and because no other program could cause the 1 % user CPU load).
> The dd runs at normal shell scheduling priority, so it should be
> preempted immediately by my test program!
out of curiosity, does running it with "nice -n 19" change anything?
> 2.) Using a realtime-preempt-2.6.12-rc1-V0.7.41-11 kernel with
> PREEMPT_RT:
> If my test program runs at rtpri 99, the problem is gone: It is
> scheduled within 30 microseconds after the rtc interrupt.
> If my test program runs at rtpri 1, it still suffers from delays
> in the 30 to 300 millisecond range.
so your problem is solved, no?
also, did you try a (plain) preemptable kernel?
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