* Re: [PATCH v2 02/12] sched/deadline: Less agressive dl_server handling
[not found] ` <20250702121158.465086194@infradead.org>
@ 2025-07-30 9:34 ` Geert Uytterhoeven
2025-07-30 9:46 ` Juri Lelli
0 siblings, 1 reply; 3+ messages in thread
From: Geert Uytterhoeven @ 2025-07-30 9:34 UTC (permalink / raw)
To: Peter Zijlstra
Cc: mingo, juri.lelli, vincent.guittot, dietmar.eggemann, rostedt,
bsegall, mgorman, vschneid, clm, linux-kernel, linux-m68k
Hi Peter,
On Wed, 2 Jul 2025 at 14:19, Peter Zijlstra <peterz@infradead.org> wrote:
> Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
> bandwidth control") caused a significant dip in his favourite
> benchmark of the day. Simply disabling dl_server cured things.
>
> His workload hammers the 0->1, 1->0 transitions, and the
> dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
> idea in hind sight and all that.
>
> Change things around to only disable the dl_server when there has not
> been a fair task around for a whole period. Since the default period
> is 1 second, this ensures the benchmark never trips this, overhead
> gone.
>
> Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
> Reported-by: Chris Mason <clm@meta.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20250520101727.507378961@infradead.org
Thanks for your patch, which is now commit cccb45d7c4295bbf
("sched/deadline: Less agressive dl_server handling") upstream.
This commit causes
sched: DL replenish lagged too much
to be printed after full user-space (Debian) start-up on m68k
(atari_defconfig running on ARAnyM). Reverting this commit and fixing
the small conflict gets rid of the message.
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -701,6 +701,7 @@ struct sched_dl_entity {
> unsigned int dl_defer : 1;
> unsigned int dl_defer_armed : 1;
> unsigned int dl_defer_running : 1;
> + unsigned int dl_server_idle : 1;
>
> /*
> * Bandwidth enforcement timer. Each -deadline task has its
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq
> /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */
> static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC;
>
> +static bool dl_server_stopped(struct sched_dl_entity *dl_se);
> +
> static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se)
> {
> struct rq *rq = rq_of_dl_se(dl_se);
> @@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti
>
> if (!dl_se->server_has_tasks(dl_se)) {
> replenish_dl_entity(dl_se);
> + dl_server_stopped(dl_se);
> return HRTIMER_NORESTART;
> }
>
> @@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r
> void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
> {
> /* 0 runtime = fair server disabled */
> - if (dl_se->dl_runtime)
> + if (dl_se->dl_runtime) {
> + dl_se->dl_server_idle = 0;
> update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
> + }
> }
>
> void dl_server_start(struct sched_dl_entity *dl_se)
> @@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent
> setup_new_dl_entity(dl_se);
> }
>
> - if (!dl_se->dl_runtime)
> + if (!dl_se->dl_runtime || dl_se->dl_server_active)
> return;
>
> dl_se->dl_server_active = 1;
> @@ -1684,6 +1689,20 @@ void dl_server_stop(struct sched_dl_enti
> dl_se->dl_server_active = 0;
> }
>
> +static bool dl_server_stopped(struct sched_dl_entity *dl_se)
> +{
> + if (!dl_se->dl_server_active)
> + return false;
> +
> + if (dl_se->dl_server_idle) {
> + dl_server_stop(dl_se);
> + return true;
> + }
> +
> + dl_se->dl_server_idle = 1;
> + return false;
> +}
> +
> void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
> dl_server_has_tasks_f has_tasks,
> dl_server_pick_f pick_task)
> @@ -2435,7 +2454,7 @@ static struct task_struct *__pick_task_d
> if (dl_server(dl_se)) {
> p = dl_se->server_pick_task(dl_se);
> if (!p) {
> - if (dl_server_active(dl_se)) {
> + if (!dl_server_stopped(dl_se)) {
> dl_se->dl_yielded = 1;
> update_curr_dl_se(rq, dl_se, 0);
> }
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5879,7 +5879,6 @@ static bool throttle_cfs_rq(struct cfs_r
> struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
> struct sched_entity *se;
> long queued_delta, runnable_delta, idle_delta, dequeue = 1;
> - long rq_h_nr_queued = rq->cfs.h_nr_queued;
>
> raw_spin_lock(&cfs_b->lock);
> /* This will start the period timer if necessary */
> @@ -5963,10 +5962,6 @@ static bool throttle_cfs_rq(struct cfs_r
>
> /* At this point se is NULL and we are at root level*/
> sub_nr_running(rq, queued_delta);
> -
> - /* Stop the fair server if throttling resulted in no runnable tasks */
> - if (rq_h_nr_queued && !rq->cfs.h_nr_queued)
> - dl_server_stop(&rq->fair_server);
> done:
> /*
> * Note: distribution will already see us throttled via the
> @@ -7060,7 +7055,6 @@ static void set_next_buddy(struct sched_
> static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
> {
> bool was_sched_idle = sched_idle_rq(rq);
> - int rq_h_nr_queued = rq->cfs.h_nr_queued;
> bool task_sleep = flags & DEQUEUE_SLEEP;
> bool task_delayed = flags & DEQUEUE_DELAYED;
> struct task_struct *p = NULL;
> @@ -7144,9 +7138,6 @@ static int dequeue_entities(struct rq *r
>
> sub_nr_running(rq, h_nr_queued);
>
> - if (rq_h_nr_queued && !rq->cfs.h_nr_queued)
> - dl_server_stop(&rq->fair_server);
> -
> /* balance early to pull high priority tasks */
> if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
> rq->next_balance = jiffies;
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
^ permalink raw reply [flat|nested] 3+ messages in thread