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

Mike Galbraith
2010-05-15T17:07:07Z

On Sat, 2010-05-15 at 14:04 +0200, Peter Zijlstra wrote:
> 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?

I was going to just skip subtracting waker's weight ala

/*
* If sync wakeup then subtract the (maximum possible)
* effect of the currently running task from the load
* of the current CPU:
*/
if (sync && !task_has_rt_policy(curr))
...

-Mike