Hello, Thanks for the tip for testing.I guess I should open a new thread because what follows is more about the result that the testing procedure.
Quick recap on my original test, I have a kernel module timer (clock monotonic, absolute) flipping a bit with some outb(val, 0x3f8 + COM_MCR)
I ran 'cyclictest' in parallel with all the load (make -jN) with a local kernel tree and another on nfs, both give similar results: cyclictest is spot on, my timer does occasional excursion.
So I looked at cyclictest and thought let's do it the same way. now I have now another cdev module giving user land access to flip COM0 with some IOCTL... and to my surprise: that does perform well.
I'm a new comer to these matters but I find it counter-intuitive my RT tasks (set priority 99) "works better" than my kernel timer. I'm looking at understanding this better. Is it just expected? some params I can set to harden things in my kernel timer? any pointers to understand this would be great.
Regards, Matthieu On 05/16/12 08:55, Clark Williams wrote:
On Tue, 15 May 2012 21:55:37 -0400 Steven Rostedt<rostedt@xxxxxxxxxxx> wrote:On Tue, 2012-05-15 at 16:08 -0700, Matthieu Bec wrote:Hello all, I was wondering what people used to check RT_PREEMPT behavior under load/stress?There is a test suite that Red Hat uses called rt-eval (I believe). Clark can give you more info on that.It's called rteval and I have a git tree here: git://git.kernel.org/pub/scm/linux/kernel/git/clrkwllms/rteval.git It's basically some python scripting to do much of what Steven describes below. When it starts up it kicks off a kernel make with 2* the number of available processors (make -j<n*2>) and runs hackbench, both in loop. Then it kicks off cyclictest to measure the system latency under load. I usually run it like this: $ sudo rteval --duration=12h At the end it summarizes the results of the run.I'm trying to test the accuracy of my timers and have a test where I setup a kernel module with an hr-timer flipping RTS bit on serial COM0 periodically, which I can look on an oscilloscope. the scope triggers on rising edge, I call jitter what shows on the falling side: under no specific load I get ~ 10 us (worst case waiting a long time) My initial idea for stressing the system was to compile a kernel, make -j 8 (#cores) that I thought would exercise CPU and IO if anything. As it happens, it's "mostly good" but I do get occasional (but repeatable) wild excursions (>100us)The tests I do is the following: I run "cyclictest -n -p 80 -t -i 250" then in another window I run a kernel compile using distcc (to stress the network as well) with make -j40, it basically does: while :; make clean; make -j40; done Then I also run hackbench (written by Rusty Russell), with: while :; hackbench 50 ; done I run the above on a single machine, while on another machine I run ktest against the -rt kernel to test different configs (with and without PREEMPT_RT enabled and such). I do this for both i386 and x86_64.Looking around, I found a tool called 'stress' - http://weather.ou.edu/~apw/projects/stress/ Under these new conditions, the system behaves really well again ~20 us stable all the way. So both tests give different result, I'm not sure which to trust. I was thinking maybe there is some weird interaction with the kernel and building the kernel that make the 'bad' test invalid? I have RT_PREEMPT 3.0.18-rt34 SMP x86_64Now, I run the above stress tests that I mentioned for several hours before I release a stable kernel. I run this on a 2.6GHz xeon core2, and I may hit at most 70us latency with cyclictest. That's a high, it usually stays below 50us. We consider>100us on this type of hardware a bug which needs to be fixed. -- Steve
-- Matthieu Bec GMTO Corp. cell: +1 626 354 9367 P.O. Box 90933 phone: +1 626 204 0527 Pasadena, CA 91109-0933 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html