Re: [RFC 4/8] Improve the tracking of active utilisation

From: Peter Zijlstra
Date: Tue Jan 19 2016 - 08:47:54 EST

On Tue, Jan 19, 2016 at 01:20:13PM +0100, Luca Abeni wrote:
> Hi Peter,
> On 01/14/2016 08:43 PM, Peter Zijlstra wrote:
> >On Thu, Jan 14, 2016 at 04:24:49PM +0100, Luca Abeni wrote:
> >>This patch implements a more theoretically sound algorithm for
> >>thracking the active utilisation: instead of decreasing it when a
> >>task blocks, use a timer (the "inactive timer", named after the
> >>"Inactive" task state of the GRUB algorithm) to decrease the
> >>active utilisaation at the so called "0-lag time".
> >
> >See also the large-ish comment in __setparam_dl().
> >
> >If we go do proper 0-lag, as GRUB requires, then we might as well use it
> >for that.
> Just to check if I understand correctly:
> I would need to remove "dl_b->total_bw -= p->dl.dl_bw;" from task_dead_dl(),
> and __dl_clear() from "else if (!dl_policy(policy) && task_has_dl_policy(p))"
> in dl_overflow(). Then, arm the inactive_timer in these cases, and add the
> __dl_clear() in the "if (!dl_task(p))" in inactive_task_timer()... Right?


> If this understanding is correct (modulo some details that I'll figure out
> during testing), I'll try this.

Yes, there's bound to be 'fun' details..

> In theory, the inactive_timer would be the right place to also decrease
> the active utilisation when a task switches from SCHED_DEADLINE to something
> else... But this is problematic if the task migrates after switching from
> SCHED_DEADLINE and before the timer fires.

urgh, yes.. details :-)