Messages in this thread | | | From | David Laight <> | Subject | RE: sched/fair: scheduler not running high priority process on idle cpu | Date | Tue, 14 Jan 2020 17:33:50 +0000 |
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From: Steven Rostedt > Sent: 14 January 2020 16:59 > > On Tue, 14 Jan 2020 16:50:43 +0000 > David Laight <David.Laight@ACULAB.COM> wrote: > > > I've a test that uses four RT priority processes to process audio data every 10ms. > > One process wakes up the other three, they all 'beaver away' clearing a queue of > > jobs and the last one to finish sleeps until the next tick. > > Usually this takes about 0.5ms, but sometimes takes over 3ms. > > > > AFAICT the processes are normally woken on the same cpu they last ran on. > > There seems to be a problem when the selected cpu is running a (low priority) > > process that is looping in kernel [1]. > > I'd expect my process to be picked up by one of the idle cpus, but this > > doesn't happen. > > Instead the process sits in state 'waiting' until the active processes sleeps > > (or calls cond_resched()). > > > > Is this really the expected behaviour????? > > It is with CONFIG_PREEMPT_VOLUNTARY. I think you want to recompile your > kernel with CONFIG_PREEMPT. The idea is that the RT task will continue > to run on the CPU it last ran on, and would push off the lower priority > task to the idle CPU. But CONFIG_PREEMPT_VOLUNTARY means that this > will have to wait for the running task to not be in kernel context or > hit a cond_resched() which is the "voluntary" scheduling point.
I have added a cond_resched() to the offending loop, but a close look implies that code is called with a lock held in another (less common) path so that can't be directly committed and so CONFIG_PREEMPT won't help.
Indeed requiring CONFIG_PREEMPT doesn't help when customers are running the application, nor (probably) on AWS since I doubt it is ever the default.
Does the same apply to non-RT tasks? I can select almost any priority, but RT ones are otherwise a lot better.
I've also seen RT processes delayed by the network stack 'bh' that runs in a softint from the hardware interrupt. That can take a while (clearing up tx and refilling rx) and I don't think we have any control over the cpu it runs on?
The cost of ftrace function call entry/exit (about 200 clocks) makes it rather unsuitable for any performance measurements unless only a very few functions are traced - which rather requires you know what the code is doing :-(
David
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