linux-kernel - Re: [PATCH -v2 15/17] sched: Fix migrate

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: <jhjr1qbmin4.mognet@arm.com>
Date:   Tue, 06 Oct 2020 17:19:43 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, qais.yousef@....com, swood@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vincent.donnefort@....com,
        tj@...nel.org
Subject: Re: [PATCH -v2 15/17] sched: Fix migrate_disable() vs rt/dl balancing


On 06/10/20 14:48, Peter Zijlstra wrote:
> On Tue, Oct 06, 2020 at 12:20:43PM +0100, Valentin Schneider wrote:
>>
>> An alternative I could see would be to prevent those piles from forming
>> altogether, say by issuing a similar push_cpu_stop() on migrate_disable()
>> if the next pushable task is already migrate_disable(); but that's a
>> proactive approach whereas yours is reactive, so I'm pretty sure that's
>> bound to perform worse.
>
> I think it is always possible to form pileups. Just start enough tasks
> such that newer, higher priority, tasks have to preempt existing tasks.
>
> Also, we might not be able to place the task elsewhere, suppose we have
> all our M CPUs filled with an RT task, then when the lowest priority
> task has migrate_disable(), wake the highest priority task.
>
> Per the SMP invariant, this new highest priority task must preempt the
> lowest priority task currently running, otherwise we would not be
> running the M highest prio tasks.
>

Right, and it goes the other way around for the migrate_disable() task: if
it becomes one of the M highest prio tasks, then it *must* run, and
push/pulling its CPU's current away is the only way to do so...

> That's not to say it might not still be beneficial from trying to avoid
> them, but we must assume a pilup will occur, therefore my focus was on
> dealing with them as best we can first.

"Funny" all that... Thanks!