Re: commit e9e9250b: sync wakeup bustage when waker is an RT task

Peter Zijlstra
2010-05-15T14:28:45Z

On Sat, 2010-05-15 at 13:57 +0200, Mike Galbraith wrote:
> Hi Peter,
>
> This commit excluded RT tasks from rq->load, was that intentional? The
> comment in struct rq states that load reflects *all* tasks, but since
> this commit, that's no longer true.

Right, because a static load value does not accurately reflect a RT task
which can run as long as it pretty well pleases. So instead we measure
the time spend running !fair tasks and scale down the cpu_power
proportionally.

> Looking at lmbench lat_udp in a PREEMPT_RT kernel, I noticed that
> wake_affine() is failing for sync wakeups when it should not. It's
> doing so because the waker in this case is an RT kernel thread
> (sirq-net-rx) - we subtract the sync waker's weight, when it was never
> added in the first place, resulting in this_load going gaga. End result
> is quite high latency numbers due to tasks jabbering cross-cache.
>
> If the exclusion was intentional, I suppose I can do a waker class check
> in wake_affine() to fix it.

So basically make all RT wakeups sync?