linux-kernel - [PATCH] sched/fair: fix calc_cfs

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Date:   Mon, 19 Dec 2016 23:40:14 +0100
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")

exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled,
and must thus be scaled before being manipulated against "shares" amounts.

Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: stable@...r.kernel.org
Fixes: 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")

---
This should be backported to 4.7 and 4.8 to fix scheduling priorities
miscalculations

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6559d19..be84f72 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2657,8 +2657,8 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
 	if (tg_weight)
 		shares /= tg_weight;
 
-	if (shares < MIN_SHARES)
-		shares = MIN_SHARES;
+	if (shares < scale_load(MIN_SHARES))
+		shares = scale_load(MIN_SHARES);
 	if (shares > tg->shares)
 		shares = tg->shares;