Messages in this thread | | | From | "Nikita V. Youshchenko" <> | Subject | [rt-preempt / powerpc] Looks like tick_sched_timer() is a major latency source | Date | Tue, 23 Jun 2009 20:55:11 +0400 |
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Hi
I was trying to get minimum possible in-kernel latencies on a freescale 8360 CPU based board (powerpc arch). There is a project where <30 us worst case latency is required, and question was - is it possible to use linux+preempt-rt (which is preferred for a number of reason) or not.
I used 2.6.29.5-rt21 kernel, which, for my best knowledge, was the last "release" of preempt-rt at the moment when I started.
I used 8360's Generic Timer Module to both to generate interrupts and to measure lanencies. GTM timers continue to count after firing an interrupt, so latency may be measured my reading current timer value and comparing that with the value at which interrupt was generated. I believe this method is both simple and accurate.
The result is:
- average latency up to IRQF_NODELAY interrupt header is 2-3 us, even on a system with high i/o and cpu load. To generate load, I used a flood ping and several 'while true; do true; done' cpu-eaters.
- but worst-case latency is BAD. I occasionally got >50 us even on idle system.
I tried hard to identify the latency source, and at some moment discovered tick_sched_timer() from kernel/time/tick-sched.c. This routine is called from timer interrupt with hardware interrupts disabled, and may execute for 50 us and more.
Could someone please comment on this? Is it possible to move (part of) tick_sched_timer() call tree out of hardware-interrupts-disabled context without breaking things?
Nikita
Please CC: reply to my e-mail address. Thanks.
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