linux-kernel - [PATCH] sched/fair: Fix fixed point arithmetic width for shares and effective load

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Message-Id: <1471874441-24701-1-git-send-email-dietmar.eggemann@arm.com>
Date:   Mon, 22 Aug 2016 15:00:41 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: Fix fixed point arithmetic width for shares and effective load

Since commit 2159197d6677 ("sched/core: Enable increased load resolution
on 64-bit kernels") we now have two different fixed point units for
load.
shares in calc_cfs_shares() has 20 bit fixed point unit on 64-bit
kernels. Therefore use scale_load() on MIN_SHARES.
wl in effective_load() has 10 bit fixed point unit. Therefore use
scale_load_down() on tg->shares which has 20 bit fixed point unit on
64-bit kernels.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---

Besides the issue with load_above_capacity when it comes to different
fixed point units for load "[PATCH] sched/fair: Fix load_above_capacity
fixed point arithmetic width" there are similar issues for shares in
calc_cfs_shares() as well as wl in effective_load().

 kernel/sched/fair.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 61d485421bed..18f80c4c7737 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2530,8 +2530,8 @@ 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;

@@ -5023,9 +5023,9 @@ static long effective_load(struct task_group *tg, int cpu, long wl, long wg)
 		 * wl = S * s'_i; see (2)
 		 */
 		if (W > 0 && w < W)
-			wl = (w * (long)tg->shares) / W;
+			wl = (w * (long)scale_load_down(tg->shares)) / W;
 		else
-			wl = tg->shares;
+			wl = scale_load_down(tg->shares);

 		/*
 		 * Per the above, wl is the new se->load.weight value; since
-- 
1.9.1