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Subject[PATCH -v2 18/18] sched/fair: Update calc_group_*() comments
I had a wee bit of trouble recalling how the calc_group_runnable()
stuff worked.. add hopefully better comments.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/sched/fair.c | 68 ++++++++++++++++++++++++++++++++++------------------
1 file changed, 45 insertions(+), 23 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2874,7 +2874,7 @@ void reweight_task(struct task_struct *p
* Now, in that special case (1) reduces to:
*
* tg->weight * grq->load.weight
- * ge->load.weight = ----------------------------- = tg>weight (4)
+ * ge->load.weight = ----------------------------- = tg->weight (4)
* grp->load.weight
*
* That is, the sum collapses because all other CPUs are idle; the UP scenario.
@@ -2888,6 +2888,18 @@ void reweight_task(struct task_struct *p
* --------------------------------------------------- (5)
* tg->load_avg - grq->avg.load_avg + grq->load.weight
*
+ * But because grq->load.weight can drop to 0, resulting in a divide by zero,
+ * we need to use grq->avg.load_avg as its lower bound, which then gives:
+ *
+ *
+ * tg->weight * grq->load.weight
+ * ge->load.weight = ----------------------------- (6)
+ * tg_load_avg'
+ *
+ * Where:
+ *
+ * tg_load_avg' = tg->load_avg - grq->avg.load_avg +
+ * max(grq->load.weight, grq->avg.load_avg)
*
* And that is shares_weight and is icky. In the (near) UP case it approaches
* (4) while in the normal case it approaches (3). It consistently
@@ -2904,10 +2916,6 @@ static long calc_group_shares(struct cfs

tg_shares = READ_ONCE(tg->shares);

- /*
- * Because (5) drops to 0 when the cfs_rq is idle, we need to use (3)
- * as a lower bound.
- */
load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);

tg_weight = atomic_long_read(&tg->load_avg);
@@ -2936,32 +2944,46 @@ static long calc_group_shares(struct cfs
}

/*
- * The runnable shares of this group are calculated as such
+ * This calculates the effective runnable weight for a group entity based on
+ * the group entity weight calculated above.
+ *
+ * Because of the above approximation (2), our group entity weight is
+ * an load_avg based ratio (3). This means that it includes blocked load and
+ * does not represent the runnable weight.
*
- * max(cfs_rq->avg.runnable_load_avg, cfs_rq->runnable_weight)
- * shares * ------------------------------------------------------------
- * max(cfs_rq->avg.load_avg, cfs_rq->load.weight)
- *
- * We do this to keep the shares in line with expected load on the cfs_rq.
- * Consider a cfs_rq that has several tasks wake up on this cfs_rq for the first
- * time, it's runnable_load_avg is not going to be representative of the actual
- * load this cfs_rq will now experience, which will bias us agaisnt this cfs_rq.
- * The weight on the cfs_rq is the immediate effect of having new tasks
- * enqueue'd onto it which should be used to calculate the new runnable shares.
- * At the same time we need the actual load_avg to be the lower bounds for the
- * calculation, to handle when our weight drops quickly from having entities
- * dequeued.
+ * Approximate the group entity's runnable weight per ratio from the group
+ * runqueue:
+ *
+ * grq->avg.runnable_load_avg
+ * ge->runnable_weight = ge->load.weight * -------------------------- (7)
+ * grq->avg.load_avg
+ *
+ * However, analogous to above, since the avg numbers are slow, this leads to
+ * transients in the from-idle case. Instead we use:
+ *
+ * ge->runnable_weight = ge->load.weight *
+ *
+ * max(grq->avg.runnable_load_avg, grq->runnable_weight)
+ * ----------------------------------------------------- (8)
+ * max(grq->avg.load_avg, grq->load.weight)
+ *
+ * Where these max() serve both to use the 'instant' values to fix the slow
+ * from-idle and avoid the /0 on to-idle, similar to (6).
*/
static long calc_group_runnable(struct cfs_rq *cfs_rq, long shares)
{
- long load_avg = max(cfs_rq->avg.load_avg,
- scale_load_down(cfs_rq->load.weight));
- long runnable = max(cfs_rq->avg.runnable_load_avg,
- scale_load_down(cfs_rq->runnable_weight));
+ long runnable, load_avg;
+
+ load_avg = max(cfs_rq->avg.load_avg,
+ scale_load_down(cfs_rq->load.weight));
+
+ runnable = max(cfs_rq->avg.runnable_load_avg,
+ scale_load_down(cfs_rq->runnable_weight));

runnable *= shares;
if (load_avg)
runnable /= load_avg;
+
return clamp_t(long, runnable, MIN_SHARES, shares);
}
# endif /* CONFIG_SMP */

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