linux-kernel - Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with expired deadline

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Message-ID: <2daa2e6217eeaa239616303626c0d73d808ae947.camel@redhat.com>
Date: Fri, 31 Oct 2025 16:41:22 +0100
From: Gabriele Monaco <gmonaco@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>, linux-kernel@...r.kernel.org, Ingo
 Molnar <mingo@...hat.com>, Clark Williams <williams@...hat.com>,
 arighi@...dia.com
Subject: Re: [RFC PATCH] sched/deadline: Avoid dl_server boosting with
 expired deadline

On Fri, 2025-10-31 at 16:20 +0100, Peter Zijlstra wrote:
> On Fri, Oct 31, 2025 at 02:24:17PM +0100, Gabriele Monaco wrote:
> > 
> > Different scenario if I have the CPU busy with other tasks (e.g. RT
> > policies), there I can see the server stopping and starting again.
> > After I do this I seem to get a different behaviour (even some boosting
> > after idle), I'm trying to understand what's going on.
> > 

After running some heavy RT workload (stress-ng --cpu 10 --sched rr) I do see
the server stopping and starting as the models would expect, but somehow it's
always boosting as soon as it's started.

Apparently dl_defer_running is always 1 in that scenario. Perhaps running idle
counts as running something too, so it never defers. But I can't really see how
this happens..

<idle>-0     12.964637: (+8)     sched_switch:         swapper/9:0 [120] R ==> irq/50:212 [49]
irq/50-212   12.964699: (+58)    sched_wakeup:         trace-cmd:544 [120] CPU:000
irq/50-212   12.964831: (+24)    sched_migrate_task:   comm=trace-cmd pid=544 prio=120 orig_cpu=0 dest_cpu=9
irq/50-212   12.964842: (+11)    event_nomiss:         -9: idle x dl_server_start -> idle
irq/50-212   12.964843: (+1)     sched_dl_server_start: comm=server pid=-9 runtime=16238881 deadline=13592407587 yielded=0
irq/50-212   12.964854: (+11)    bprint:               pick_task_dl: Picking server 9 (armed 1 running 1)
irq/50-212   12.964861: (+7)     event_nomiss:         -9: idle x sched_switch_in -> running
irq/50-212   12.964862: (+1)     sched_switch:         irq/50:212 [49] S ==> trace-cmd:544 [120]

The trace-cmd in the example above is boosted, like highlighted by that bprint	
just before.

> Ooooh, because idle time is accounted against the server budget too.
> 
> That is, idle and running fair are both [2] in my comment. So we never
> get to [5].

Yeah that must be it, however I don't think it explains the other curious
behaviour above.

> Humm, let me noodle a bit more on this. Also, I should see if I can get
> graphviz to draw ascii art state diagrams :-)

Dot doesn't do it but graph-easy does reasonably well.

Thanks,
Gabriele