linux-kernel - Re: [PATCH v3 4/6] sched/fair: Limit run to parity to the min slice of enqueued entities

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: <CAKfTPtBhkBTcWW88QV2tUFEF_kvHZpai7OvHG3q5qU9shpj-3g@mail.gmail.com>
Date: Mon, 21 Jul 2025 11:06:15 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, dhaval@...nis.ca, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/6] sched/fair: Limit run to parity to the min slice
 of enqueued entities

Hi Madadi,

Sorry for the late reply but I have limited network access at the moment.

On Sun, 13 Jul 2025 at 20:17, Madadi Vineeth Reddy
<vineethr@...ux.ibm.com> wrote:
>
> Hi Vincent, Peter
>
> On 10/07/25 18:04, Peter Zijlstra wrote:
> >
> >>> If I set my task’s custom slice to a larger value but another task has a smaller slice,
> >>> this change will cap my protected window to the smaller slice. Does that mean my custom
> >>> slice is no longer honored?
> >>
> >> What do you mean by honored ? EEVDF never mandates that a request of
> >> size slice will be done in one go. Slice mainly defines the deadline
> >> and orders the entities but not that it will always run your slice in
> >> one go. Run to parity tries to minimize the number of context switches
> >> between runnable tasks but must not break fairness and lag theorem.So
> >> If your task A has a slice of 10ms and task B wakes up with a slice of
> >> 1ms. B will preempt A because its deadline is earlier. If task B still
> >> wants to run after its slice is exhausted, it will not be eligible and
> >> task A will run until task B becomes eligible, which is as long as
> >> task B's slice.
> >
> > Right. Added if you don't want wakeup preemption, we've got SCHED_BATCH
> > for you.
>
> Thanks for the explanation. Understood now that slice is only for deadline
> calculation and ordering for eligible tasks.
>
> Before your patch, I observed that each task ran for its full custom slice
> before preemption, which led me to assume that slice directly controlled
> uninterrupted runtime.
>
> With the patch series applied and RUN_TO_PARITY=true, I now see the expected behavior:
> - Default slice (~2.8 ms): tasks run ~3 ms each.
> - Increasing one task’s slice doesn’t extend its single‐run duration—it remains ~3 ms.
> - Decreasing one tasks’ slice shortens everyone’s run to that new minimum.
>
> With this patch series, With NO_RUN_TO_PARITY, I see runtimes near 1 ms (CONFIG_HZ=1000).
>
> However, without your patches, I was still seeing ~3 ms runs even with NO_RUN_TO_PARITY,
> which confused me because I expected runtime to drop to ~1 ms (preempt at every tick)
> rather than run up to the default slice.
>
> Without your patches and having RUN_TO_PARITY is as expected. Task running till it's
> slice when eligible.
>
> I ran these with 16 stress‑ng threads pinned to one CPU.
>
> Please let me know if my understanding is incorrect, and why I was still seeing ~3 ms
> runtimes with NO_RUN_TO_PARITY before this patch series.

Before my patchset both NO_RUN_TO_PARITY and RUN_TO_PARITY were wrong.
Patch 2 fixes NO_RUN_TO_PARITY and others RUN_TO_PARITY

>
> Thanks,
> Madadi Vineeth Reddy