linux-kernel - Re: [PATCH v2] sched: Don't try to catch up excess steal time.

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: <20240912144727.GC1578937@linux.ibm.com>
Date: Thu, 12 Sep 2024 20:17:27 +0530
From: Srikar Dronamraju <srikar@...ux.ibm.com>
To: Suleiman Souhlal <suleiman@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, joelaf@...gle.com,
        vineethrp@...gle.com, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, ssouhlal@...ebsd.org,
        David Woodhouse <dwmw2@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2] sched: Don't try to catch up excess steal time.

* Suleiman Souhlal <suleiman@...gle.com> [2024-09-11 20:15:22]:

> When steal time exceeds the measured delta when updating clock_task, we
> currently try to catch up the excess in future updates.
> However, this results in inaccurate run times for the future things using
> clock_task, as they end up getting additional steal time that did not
> actually happen.
> 
> For example, suppose a task in a VM runs for 10ms and had 15ms of steal
> time reported while it ran. clock_task rightly doesn't advance. Then, a
> different taks runs on the same rq for 10ms without any time stolen in
> the host.
> Because of the current catch up mechanism, clock_sched inaccurately ends
> up advancing by only 5ms instead of 10ms even though there wasn't any
> actual time stolen. The second task is getting charged for less time
> than it ran, even though it didn't deserve it.
> This can result in tasks getting more run time than they should actually
> get.
> 
> So, we instead don't make future updates pay back past excess stolen time.
> 
> Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
> ---
> v2:
> - Slightly changed to simply moving one line up instead of adding
>   new variable.
> 
> v1: https://lore.kernel.org/lkml/20240806111157.1336532-1-suleiman@google.com
> ---

After moving the line up, this looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@...ux.ibm.com>

-- 
Thanks and Regards
Srikar Dronamraju