linux-kernel - Re: [tip:sched/eevdf] [sched/fair] e0c2ff903c: phoronix-test-suite.blogbench.Write.final

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Date:   Wed, 16 Aug 2023 15:40:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Galbraith <umgwanakikbuti@...il.com>
Cc:     Chen Yu <yu.c.chen@...el.com>,
        kernel test robot <oliver.sang@...el.com>,
        oe-lkp@...ts.linux.dev, lkp@...el.com,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [tip:sched/eevdf] [sched/fair]  e0c2ff903c:
 phoronix-test-suite.blogbench.Write.final_score -34.8% regression

On Wed, Aug 16, 2023 at 02:37:16PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 14, 2023 at 08:32:55PM +0200, Mike Galbraith wrote:
> 
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -875,6 +875,12 @@ static struct sched_entity *pick_eevdf(s
> >  	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
> >  		curr = NULL;
> >  
> > +	/*
> > +	 * Once selected, run the task to parity to avoid overscheduling.
> > +	 */
> > +	if (sched_feat(RUN_TO_PARITY) && curr)
> > +		return curr;
> > +
> >  	while (node) {
> >  		struct sched_entity *se = __node_2_se(node);
> >  
> 
> So I read it wrong last night... but I rather like this idea. But
> there's something missing. When curr starts a new slice it should
> probably do a full repick and not stick with it.
> 
> Let me poke at this a bit.. nice

Something like so.. it shouldn't matter much now, but might make a
difference once we start mixing different slice lengths.

---
 kernel/sched/fair.c     | 12 ++++++++++++
 kernel/sched/features.h |  1 +
 2 files changed, 13 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fe5be91c71c7..128a78f3f264 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -873,6 +873,13 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
 	if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
 		curr = NULL;
 
+	/*
+	 * Once selected, run a task until it either becomes non-eligible or
+	 * until it gets a new slice. See the HACK in set_next_entity().
+	 */
+	if (sched_feat(RUN_TO_PARITY) && curr && curr->vlag == curr->deadline)
+		return curr;
+
 	while (node) {
 		struct sched_entity *se = __node_2_se(node);
 
@@ -5168,6 +5175,11 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 		update_stats_wait_end_fair(cfs_rq, se);
 		__dequeue_entity(cfs_rq, se);
 		update_load_avg(cfs_rq, se, UPDATE_TG);
+		/*
+		 * HACK, stash a copy of deadline at the point of pick in vlag,
+		 * which isn't used until dequeue.
+		 */
+		se->vlag = se->deadline;
 	}
 
 	update_stats_curr_start(cfs_rq, se);
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 61bcbf5e46a4..f770168230ae 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -6,6 +6,7 @@
  */
 SCHED_FEAT(PLACE_LAG, true)
 SCHED_FEAT(PLACE_DEADLINE_INITIAL, true)
+SCHED_FEAT(RUN_TO_PARITY, true)
 
 /*
  * Prefer to schedule the task we woke last (assuming it failed