Re: [PATCH v4 1/2] sched/deadline: Fix bandwidth reclaim equation in GRUB

From: Joel Fernandes
Date: Fri Jun 02 2023 - 14:40:27 EST

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On Fri, May 26, 2023 at 10:55 AM Vineeth Pillai <vineeth@xxxxxxxxxxxxxxx> 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)

Makes sense, your patch fixes the issue where we need the depreciation
ratio to factor in the throttling limits as well.

The code looks sane to me too.

Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

- Joel

>
> 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>
> ---
> kernel/sched/deadline.c | 56 +++++++++++++++++++----------------------
> kernel/sched/sched.h | 6 +++++
> 2 files changed, 32 insertions(+), 30 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 71b24371a6f7..ea5db080f008 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1260,43 +1260,39 @@ int dl_runtime_exceeded(struct sched_dl_entity *dl_se)
> }
>
> /*
> - * This function implements the GRUB accounting rule:
> - * according to the GRUB reclaiming algorithm, the runtime is
> - * not decreased as "dq = -dt", but as
> - * "dq = -max{u / Umax, (1 - Uinact - Uextra)} dt",
> + * This function implements the GRUB accounting rule. According to the
> + * GRUB reclaiming algorithm, the runtime is not decreased as "dq = -dt",
> + * but as "dq = -(max{u, (1 - Uinact - Uextra)} / Umax) dt",
> * where u is the utilization of the task, Umax is the maximum reclaimable
> - * utilization, Uinact is the (per-runqueue) inactive utilization, computed
> - * as the difference between the "total runqueue utilization" and the
> - * runqueue active utilization, and Uextra is the (per runqueue) extra
> - * reclaimable utilization.
> - * Since rq->dl.running_bw and rq->dl.this_bw contain utilizations
> - * multiplied by 2^BW_SHIFT, the result has to be shifted right by
> - * BW_SHIFT.
> - * Since rq->dl.bw_ratio contains 1 / Umax multiplied by 2^RATIO_SHIFT,
> - * dl_bw is multiped by rq->dl.bw_ratio and shifted right by RATIO_SHIFT.
> - * Since delta is a 64 bit variable, to have an overflow its value
> - * should be larger than 2^(64 - 20 - 8), which is more than 64 seconds.
> - * So, overflow is not an issue here.
> + * bandwidth, Uinact is the (per-runqueue) inactive utilization, computed as
> + * the difference between the "total runqueue utilization" and the runqueue
> + * active utilization, and Uextra is the (per runqueue) extra reclaimable
> + * utilization.
> + * Since rq->dl.running_bw and rq->dl.this_bw contain utilizations multiplied
> + * by 2^BW_SHIFT, the result has to be shifted right by BW_SHIFT.
> + * Since rq->dl.bw_ratio contains 1 / Umax multiplied by 2^RATIO_SHIFT, dl_bw
> + * is multiped by rq->dl.bw_ratio and shifted right by RATIO_SHIFT.i
> + * Since delta is a 64 bit variable, to have an overflow its value should be
> + * larger than 2^(64 - 20 - 8), which is more than 64 seconds. So, overflow is
> + * not an issue here.
> */
> static u64 grub_reclaim(u64 delta, struct rq *rq, struct sched_dl_entity *dl_se)
> {
> - u64 u_inact = rq->dl.this_bw - rq->dl.running_bw; /* Utot - Uact */
> u64 u_act;
> - u64 u_act_min = (dl_se->dl_bw * rq->dl.bw_ratio) >> RATIO_SHIFT;
> + u64 u_inact = rq->dl.this_bw - rq->dl.running_bw; /* Utot - Uact */
>
> /*
> - * Instead of computing max{u * bw_ratio, (1 - u_inact - u_extra)},
> - * we compare u_inact + rq->dl.extra_bw with
> - * 1 - (u * rq->dl.bw_ratio >> RATIO_SHIFT), because
> - * u_inact + rq->dl.extra_bw can be larger than
> - * 1 * (so, 1 - u_inact - rq->dl.extra_bw would be negative
> - * leading to wrong results)
> + * Instead of computing max{u, (u_max - u_inact - u_extra)}, we
> + * compare u_inact + u_extra with u_max - u, because u_inact + u_extra
> + * can be larger than u_max. So, u_max - u_inact - u_extra would be
> + * negative leading to wrong results.
> */
> - if (u_inact + rq->dl.extra_bw > BW_UNIT - u_act_min)
> - u_act = u_act_min;
> + if (u_inact + rq->dl.extra_bw > rq->dl.max_bw - dl_se->dl_bw)
> + u_act = dl_se->dl_bw;
> else
> - u_act = BW_UNIT - u_inact - rq->dl.extra_bw;
> + u_act = rq->dl.max_bw - u_inact - rq->dl.extra_bw;
>
> + u_act = (u_act * rq->dl.bw_ratio) >> RATIO_SHIFT;
> return (delta * u_act) >> BW_SHIFT;
> }
>
> @@ -2784,12 +2780,12 @@ static void init_dl_rq_bw_ratio(struct dl_rq *dl_rq)
> {
> if (global_rt_runtime() == RUNTIME_INF) {
> dl_rq->bw_ratio = 1 << RATIO_SHIFT;
> - dl_rq->extra_bw = 1 << BW_SHIFT;
> + dl_rq->max_bw = dl_rq->extra_bw = 1 << BW_SHIFT;
> } else {
> dl_rq->bw_ratio = to_ratio(global_rt_runtime(),
> global_rt_period()) >> (BW_SHIFT - RATIO_SHIFT);
> - dl_rq->extra_bw = to_ratio(global_rt_period(),
> - global_rt_runtime());
> + dl_rq->max_bw = dl_rq->extra_bw =
> + to_ratio(global_rt_period(), global_rt_runtime());
> }
> }
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 3e8df6d31c1e..73027c2806dc 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -753,6 +753,12 @@ struct dl_rq {
> u64 this_bw;
> u64 extra_bw;
>
> + /*
> + * Maximum available bandwidth for reclaiming by SCHED_FLAG_RECLAIM
> + * tasks of this rq. Used in calculation of reclaimable bandwidth(GRUB).
> + */
> + u64 max_bw;
> +
> /*
> * Inverse of the fraction of CPU utilization that can be reclaimed
> * by the GRUB algorithm.
> --
> 2.40.1
>

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