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

From: Luca Abeni
Date: Tue Jan 19 2016 - 07:20:23 EST

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.

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.