Re: [PATCH v5 1/2] sched/deadline: Fix bandwidth reclaim equation in GRUB

[Juri Lelli]

Hi!

On 30/05/23 09:55, 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.
> 
> Signed-off-by: Vineeth Pillai (Google) <vineeth@xxxxxxxxxxxxxxx>
> ---

This looks good to me too. Thanks a lot for working on this and of
course to Luca and Daniel who reviewed and played with it as well.

Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Best,
Juri