63 lines
2.4 KiB
Diff
63 lines
2.4 KiB
Diff
From e02cbdc12bf63da363d7e3391376819241d67fbe Mon Sep 17 00:00:00 2001
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From: Peter Zijlstra <peterz@infradead.org>
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Date: Tue, 28 Jan 2025 15:39:49 +0100
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Subject: sched/fair: Adhere to place_entity() constraints
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Mike reports that commit 6d71a9c61604 ("sched/fair: Fix EEVDF entity
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placement bug causing scheduling lag") relies on commit 4423af84b297
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("sched/fair: optimize the PLACE_LAG when se->vlag is zero") to not
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trip a WARN in place_entity().
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What happens is that the lag of the very last entity is 0 per
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definition -- the average of one element matches the value of that
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element. Therefore place_entity() will match the condition skipping
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the lag adjustment:
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if (sched_feat(PLACE_LAG) && cfs_rq->nr_queued && se->vlag) {
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Without the 'se->vlag' condition -- it will attempt to adjust the zero
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lag even though we're inserting into an empty tree.
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Notably, we should have failed the 'cfs_rq->nr_queued' condition, but
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don't because they didn't get updated.
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Additionally, move update_load_add() after placement() as is
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consistent with other place_entity() users -- this change is
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non-functional, place_entity() does not use cfs_rq->load.
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Fixes: 6d71a9c61604 ("sched/fair: Fix EEVDF entity placement bug causing scheduling lag")
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Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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Reported-by: Mike Galbraith <efault@gmx.de>
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Signed-off-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
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Signed-off-by: Mike Galbraith <efault@gmx.de>
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Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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Cc: stable@vger.kernel.org
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Link: https://lore.kernel.org/r/c216eb4ef0e0e0029c600aefc69d56681cee5581.camel@gmx.de
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---
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kernel/sched/fair.c | 4 +++-
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1 file changed, 3 insertions(+), 1 deletion(-)
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--- a/kernel/sched/fair.c
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+++ b/kernel/sched/fair.c
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@@ -3808,6 +3808,7 @@ static void reweight_entity(struct cfs_r
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update_entity_lag(cfs_rq, se);
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se->deadline -= se->vruntime;
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se->rel_deadline = 1;
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+ cfs_rq->nr_queued--;
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if (!curr)
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__dequeue_entity(cfs_rq, se);
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update_load_sub(&cfs_rq->load, se->load.weight);
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@@ -3834,10 +3835,11 @@ static void reweight_entity(struct cfs_r
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enqueue_load_avg(cfs_rq, se);
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if (se->on_rq) {
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- update_load_add(&cfs_rq->load, se->load.weight);
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place_entity(cfs_rq, se, 0);
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+ update_load_add(&cfs_rq->load, se->load.weight);
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if (!curr)
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__enqueue_entity(cfs_rq, se);
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+ cfs_rq->nr_queued++;
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/*
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* The entity's vruntime has been adjusted, so let's check
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