sched/fair: Explain why MIN_SHARES isn't scaled in calc_cfs_shares()

Signed-off-by: Dietmar Eggemann <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Samuel Thibault <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: deggeman

kernel/sched/fair.c

@@ -2657,6 +2657,18 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
 	if (tg_weight)
 		shares /= tg_weight;
 
+	/*
+	 * MIN_SHARES has to be unscaled here to support per-CPU partitioning
+	 * of a group with small tg->shares value. It is a floor value which is
+	 * assigned as a minimum load.weight to the sched_entity representing
+	 * the group on a CPU.
+	 *
+	 * E.g. on 64-bit for a group with tg->shares of scale_load(15)=15*1024
+	 * on an 8-core system with 8 tasks each runnable on one CPU shares has
+	 * to be 15*1024*1/8=1920 instead of scale_load(MIN_SHARES)=2*1024. In
+	 * case no task is runnable on a CPU MIN_SHARES=2 should be returned
+	 * instead of 0.
+	 */
 	if (shares < MIN_SHARES)
 		shares = MIN_SHARES;
 	if (shares > tg->shares)