linux-kernel - Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into SCHED

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Date:   Fri, 21 May 2021 16:04:18 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Will Deacon <will@...nel.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, kernel-team@...roid.com
Subject: Re: [PATCH v6 13/21] sched: Admit forcefully-affined tasks into
 SCHED_DEADLINE

On 21/05/21 13:02, Quentin Perret wrote:

...

> So I think Will has a point since, IIRC, the root domains get rebuilt
> during hotplug. So you can imagine a case with a single root domain, but
> CPUs 4-7 are offline. In this case, sched_setattr() will happily promote
> a task to DL as long as its affinity mask is a superset of the rd span,
> but things may get ugly when CPUs are plugged back in later on.
> 
> This looks like an existing bug though. I just tried the following on a
> system with 4 CPUs:
> 
>     // Create a task affined to CPU [0-2]
>     > while true; do echo "Hi" > /dev/null; done &
>     [1] 560
>     > mypid=$!
>     > taskset -p 7 $mypid
>     pid 560's current affinity mask: f
>     pid 560's new affinity mask: 7
> 
>     // Try to move it DL, this should fail because of the affinity
>     > chrt -d -T 5000000 -P 16666666 -p 0 $mypid
>     chrt: failed to set pid 560's policy: Operation not permitted
> 
>     // Offline CPU 3, so the rd now covers CPUs 0-2 only
>     > echo 0 > /sys/devices/system/cpu/cpu3/online
>     [  400.843830] CPU3: shutdown
>     [  400.844100] psci: CPU3 killed (polled 0 ms)
> 
>     // Try to admit the task again, which now succeeds
>     > chrt -d -T 5000000 -P 16666666 -p 0 $mypid
> 
>     // Plug CPU3 back online
>     > echo 1 > /sys/devices/system/cpu/cpu3/online
>     [  408.819337] Detected PIPT I-cache on CPU3
>     [  408.819642] GICv3: CPU3: found redistributor 3 region 0:0x0000000008100000
>     [  408.820165] CPU3: Booted secondary processor 0x0000000003 [0x410fd083]
> 
> I don't see any easy way to fix this w/o iterating over all deadline
> tasks in the rd when hotplugging a CPU back on, and blocking the hotplug
> operation if it'll cause affinity issues. Urgh.
> 

Yeah this looks like a plain existing bug, joy. :)

We fixed a few around AC lately, but I guess work wasn't complete.

Thanks,
Juri