On 12/15/2015 01:38 PM, Peter Zijlstra wrote:
On Mon, Dec 14, 2015 at 10:31:13PM +0100, Luca Abeni wrote:
There 'might' be smart pants ways around this, where you run part of
the execution at lower speed and switch to a higher speed to 'catch'
up if you exceed some boundary, such that, on average, you run at the
same speed the WCET mandates, but I'm not sure that's worth it.
Juri/Luca might know.
Some previous works (see for example
https://www.researchgate.net/profile/Giuseppe_Lipari/publication/220800940_Using_resource_reservation_techniques_for_power-aware_scheduling/links/09e41513639b2703fc000000.pdf
) investigated the usage of the "active utilisation" for switching the
CPU frequency. This "active utilisation tracking" mechanism is the same
I mentioned in the previous email, and implemented here:
https://github.com/lucabe72/linux-reclaiming/commit/49fc786a1c453148625f064fa38ea538470df55b
.
I have stuck the various PDFs and commits you've linked into my todo
list ;-) Thanks!
You are welcome :)
I suspect the "inactive timer" I used to decrease the utilisation at
the so called 0-lag time might be problematic, but I did not find any
way to implement (or approximate) the active utilisation tracking
without this timer... Anyway, if there is interest I am willing to
adapt/rework/modify my patches as needed.
So I remember something else from the BFQ code, which also had to track
entries for the 0-lag stuff, and I just had a quick peek at that code
again. And what they appear to do is keep inactive entries with a lag
deficit in a separate tree (the idle tree).
And every time they update the vtime, they also push fwd the idle tree
and expire entries on that.
I am not sure if I understand correctly the idea (I do not know the BFQ
code; I'll have a look), but I think I tried something similar:
- When a task blocks, instead of arming the inactive timer I can insert
the task in an "active non contending" tree (to use GRUB terminology)
- So, when some sched deadline function is invoked, I check the "0-lag
time" of the first task in the "active non contending" tree, and if
that time is passed I remove the task from the tree and adjust the
active utilisation
The resulting code ended up being more complex (basically, I needed to
handle the "active non contending" tree and to check it in task_tick_dl()
and update_curr_dl()). But maybe I did it wrong... I'll try this approach
again, after looking ad the BFQ code.
Thanks,
Luca
Or that is what I can make of it in a quick few minutes staring at that
code -- look for bfq_forget_idle().
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