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Date:   Thu, 1 Jun 2023 12:34:47 +0200
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Subject: Re: [PATCH v5 1/2] sched/deadline: Fix bandwidth reclaim equation in
 GRUB
To:     Vineeth Pillai <vineeth@bitbyteword.org>,
        luca.abeni@santannapisa.it, Juri Lelli <juri.lelli@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>
Cc:     Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        youssefesmat@google.com,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Ben Segall <bsegall@google.com>
References: <20230530135526.2385378-1-vineeth@bitbyteword.org>
Content-Language: en-US
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
In-Reply-To: <20230530135526.2385378-1-vineeth@bitbyteword.org>
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On 5/30/23 15:55, Vineeth Pillai wrote:
> According to the GRUB[1] rule, the runtime is depreciated as:
>   "dq = -max{u, (1 - Uinact - Uextra)} dt" (1)
> 
> To guarantee that deadline tasks doesn't starve lower class tasks,
> we do not allocate the full bandwidth of the cpu to deadline tasks.
> Maximum bandwidth usable by deadline tasks is denoted by "Umax".
> Considering Umax, equation (1) becomes:
>   "dq = -(max{u, (Umax - Uinact - Uextra)} / Umax) dt" (2)
> 
> Current implementation has a minor bug in equation (2), which this
> patch fixes.
> 
> The reclamation logic is verified by a sample program which creates
> multiple deadline threads and observing their utilization. The tests
> were run on an isolated cpu(isolcpus=3) on a 4 cpu system.
> 
> Tests on 6.3.0
> ==============
> 
> RUN 1: runtime=7ms, deadline=period=10ms, RT capacity = 95%
> TID[693]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 93.33
> TID[693]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 93.35
> 
> RUN 2: runtime=1ms, deadline=period=100ms, RT capacity = 95%
> TID[708]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 16.69
> TID[708]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 16.69
> 
> RUN 3: 2 tasks
>   Task 1: runtime=1ms, deadline=period=10ms
>   Task 2: runtime=1ms, deadline=period=100ms
> TID[631]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 62.67
> TID[632]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 6.37
> TID[631]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 62.38
> TID[632]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 6.23
> 
> As seen above, the reclamation doesn't reclaim the maximum allowed
> bandwidth and as the bandwidth of tasks gets smaller, the reclaimed
> bandwidth also comes down.
> 
> Tests with this patch applied
> =============================
> 
> RUN 1: runtime=7ms, deadline=period=10ms, RT capacity = 95%
> TID[608]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 95.19
> TID[608]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 95.16
> 
> RUN 2: runtime=1ms, deadline=period=100ms, RT capacity = 95%
> TID[616]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 95.27
> TID[616]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 95.21
> 
> RUN 3: 2 tasks
>   Task 1: runtime=1ms, deadline=period=10ms
>   Task 2: runtime=1ms, deadline=period=100ms
> TID[620]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 86.64
> TID[621]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 8.66
> TID[620]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 86.45
> TID[621]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 8.73
> 
> Running tasks on all cpus allowing for migration also showed that
> the utilization is reclaimed to the maximum. Running 10 tasks on
> 3 cpus SCHED_FLAG_RECLAIM - top shows:
> %Cpu0  : 94.6 us,  0.0 sy,  0.0 ni,  5.4 id,  0.0 wa
> %Cpu1  : 95.2 us,  0.0 sy,  0.0 ni,  4.8 id,  0.0 wa
> %Cpu2  : 95.8 us,  0.0 sy,  0.0 ni,  4.2 id,  0.0 wa
> 
> [1]: Abeni, Luca & Lipari, Giuseppe & Parri, Andrea & Sun, Youcheng.
>      (2015). Parallel and sequential reclaiming in multicore
>      real-time global scheduling.

So, I did some testing, mainly to validate the "one task cannot run on
two CPUs,"  that is, a case in which a higher utilization task would
always have its % of CPU available, even in the presence of low utilization
trying to reclaim all the CPU time. E.g.,

	Task 1: runtime=1ms, deadline=period=10ms with reclaim
	Task 2: runtime=1ms, deadline=period=10ms with reclaim
	Task 3: runtime 8ms, deadline=period=10ms without reclaim

On two CPUs task 3 always have 80% available... the other tasks
do not get 95% because of migrations.

With 3+ cpus, the tasks can run up to 95% because there are CPUs to
everyone.

I played with migrate disable to force timelines... and yet, I failed
to break things so... we are good :-).

Reviewed-by: Daniel Bristot de Oliveira <bristot@kernel.org>

Thanks!
-- Daniel