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Date:   Mon, 23 Nov 2020 09:41:23 +1100
From:   Balbir Singh <bsingharora@gmail.com>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Aaron Lu <aaron.lwe@gmail.com>,
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Subject: Re: [PATCH -tip 10/32] sched: Fix priority inversion of cookied task
 with sibling
Message-ID: <20201122224123.GE110669@balbir-desktop>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-11-joel@joelfernandes.org>
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On Tue, Nov 17, 2020 at 06:19:40PM -0500, Joel Fernandes (Google) wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> The rationale is as follows. In the core-wide pick logic, even if
> need_sync == false, we need to go look at other CPUs (non-local CPUs) to
> see if they could be running RT.
> 
> Say the RQs in a particular core look like this:
> Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task.
> 
> rq0            rq1
> CFS1 (tagged)  RT1 (not tag)
> CFS2 (tagged)
> 
> Say schedule() runs on rq0. Now, it will enter the above loop and
> pick_task(RT) will return NULL for 'p'. It will enter the above if() block
> and see that need_sync == false and will skip RT entirely.
> 
> The end result of the selection will be (say prio(CFS1) > prio(CFS2)):
> rq0             rq1
> CFS1            IDLE
> 
> When it should have selected:
> rq0             r1
> IDLE            RT
> 
> Joel saw this issue on real-world usecases in ChromeOS where an RT task
> gets constantly force-idled and breaks RT. Lets cure it.
> 
> NOTE: This problem will be fixed differently in a later patch. It just
>       kept here for reference purposes about this issue, and to make
>       applying later patches easier.
>

The changelog is hard to read, it refers to above if(), whereas there
is no code snippet in the changelog. Also, from what I can see following
the series, p->core_cookie is not yet set anywhere (unless I missed it),
so fixing it in here did not make sense just reading the series.

Balbir