linux-kernel - Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <xhsmhjzgzt2er.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Fri, 02 Aug 2024 16:39:08 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, linux-kernel@...r.kernel.org
Cc: kprateek.nayak@....com, wuyun.abel@...edance.com,
 youssefesmat@...omium.org, tglx@...utronix.de, efault@....de
Subject: Re: [PATCH 17/24] sched/fair: Implement delayed dequeue


On 27/07/24 12:27, Peter Zijlstra wrote:
> Extend / fix 86bfbb7ce4f6 ("sched/fair: Add lag based placement") by
> noting that lag is fundamentally a temporal measure. It should not be
> carried around indefinitely.
>
> OTOH it should also not be instantly discarded, doing so will allow a
> task to game the system by purposefully (micro) sleeping at the end of
> its time quantum.
>
> Since lag is intimately tied to the virtual time base, a wall-time
> based decay is also insufficient, notably competition is required for
> any of this to make sense.
>
> Instead, delay the dequeue and keep the 'tasks' on the runqueue,
> competing until they are eligible.
>
> Strictly speaking, we only care about keeping them until the 0-lag
> point, but that is a difficult proposition, instead carry them around
> until they get picked again, and dequeue them at that point.
>

Question from a lazy student who just caught up to the current state of
EEVDF...

IIUC this makes it so time spent sleeping increases an entity's lag, rather
than it being frozen & restored via the place_entity() magic.

So entities with negative lag get closer to their 0-lag point, after which
they can get picked & dequeued if still not runnable.

However, don't entities with positive lag get *further* away from their
0-lag point?