On Tue, Dec 06, 2016 at 03:20:55PM +1100, Paul Mackerras wrote: > On Tue, Dec 06, 2016 at 03:32:13AM +0100, Frederic Weisbecker wrote: > > This follows up Martin Schwidefsky's patch which propose to delay > > cputime accounting to the tick in order to minimize the calls to > > account_system_time() and alikes as these functions can carry quite some > > overhead: > > > > http://lkml.kernel.org/r/20161121111728.13a0a3db@mschwide > > > > The set includes Martin's patch, rebased on top of tip:sched/core and > > latest s390 changes, and extends it to the other implementations of > > CONFIG_VIRT_CPU_ACCOUNTING_NATIVE (powerpc and ia64) along with a few > > core changes to adapt the whole. > > > > Only built-tested though as I don't have access to any of these archs. > > The patches look reasonable at a quick look. I assume that to test > them, we would want to run a guest in an overcommitted system, so as > to get some steal time. Do you have any more specific suggestions as > to what to run as a test? Just run some benchmark and see if the > user/system/irq times look reasonable? Or do you have something more > quantitative?
So I guess we want to test both correctness and performance. To check correctness I use two little programs, one that does a userspace loop: int main(int argc, char **argv) { while (1); return 0; } And another that does a kernelspace loop. The latter is not 100% kernel loop but spends most of its time in kernel mode. int main(int argc, char **argv) { void *addr = sbrk(0); while (1) { brk(addr + 4096); brk(addr); } return 0; } Testing idle time just consist in checking the difference between two cat /proc/stat in a given timelapse for an idle CPU. For irqs it gets harder. There you just need to check if the numbers are reasonable. Now in order to measure performance, I think you need a workload that either does a lot of guest/host switch or does a lot of IRQs. Maybe just something that involves networking. Then comparing stime, hardirq and softirq should show some better nummbers. In order to increase the effect, you can set a very low HZ value (100?). Thanks.