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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Qais Yousef <qais.yousef@arm.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Quentin Perret <qperret@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.4 103/340] sched/eas: Dont update misfit status if the task is pinned
Date:   Mon,  1 Mar 2021 17:10:47 +0100
Message-Id: <20210301161053.397919838@linuxfoundation.org>
In-Reply-To: <20210301161048.294656001@linuxfoundation.org>
References: <20210301161048.294656001@linuxfoundation.org>
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From: Qais Yousef <qais.yousef@arm.com>

[ Upstream commit 0ae78eec8aa64e645866e75005162603a77a0f49 ]

If the task is pinned to a cpu, setting the misfit status means that
we'll unnecessarily continuously attempt to migrate the task but fail.

This continuous failure will cause the balance_interval to increase to
a high value, and eventually cause unnecessary significant delays in
balancing the system when real imbalance happens.

Caught while testing uclamp where rt-app calibration loop was pinned to
cpu 0, shortly after which we spawn another task with high util_clamp
value. The task was failing to migrate after over 40ms of runtime due to
balance_interval unnecessary expanded to a very high value from the
calibration loop.

Not done here, but it could be useful to extend the check for pinning to
verify that the affinity of the task has a cpu that fits. We could end
up in a similar situation otherwise.

Fixes: 3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance type")
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Quentin Perret <qperret@google.com>
Acked-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210119120755.2425264-1-qais.yousef@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3dd7c10d6a582..611adca1e6d0c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3814,7 +3814,7 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
 	if (!static_branch_unlikely(&sched_asym_cpucapacity))
 		return;
 
-	if (!p) {
+	if (!p || p->nr_cpus_allowed == 1) {
 		rq->misfit_task_load = 0;
 		return;
 	}
-- 
2.27.0