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sched/fair: Add comment to calc_cfs_shares()

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Explain the magic equation in calc_cfs_shares() a bit better.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Peter Zijlstra 2017-05-09 11:04:07 +02:00 committed by Ingo Molnar
parent 7c80cfc99b
commit cef27403cb
1 changed files with 61 additions and 0 deletions
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61
kernel/sched/fair.c
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@ -2694,6 +2694,67 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
#ifdef CONFIG_FAIR_GROUP_SCHED
# ifdef CONFIG_SMP
/*
* All this does is approximate the hierarchical proportion which includes that
* global sum we all love to hate.
*
* That is, the weight of a group entity, is the proportional share of the
* group weight based on the group runqueue weights. That is:
*
* tg->weight * grq->load.weight
* ge->load.weight = ----------------------------- (1)
* \Sum grq->load.weight
*
* Now, because computing that sum is prohibitively expensive to compute (been
* there, done that) we approximate it with this average stuff. The average
* moves slower and therefore the approximation is cheaper and more stable.
*
* So instead of the above, we substitute:
*
* grq->load.weight -> grq->avg.load_avg (2)
*
* which yields the following:
*
* tg->weight * grq->avg.load_avg
* ge->load.weight = ------------------------------ (3)
* tg->load_avg
*
* Where: tg->load_avg ~= \Sum grq->avg.load_avg
*
* That is shares_avg, and it is right (given the approximation (2)).
*
* The problem with it is that because the average is slow -- it was designed
* to be exactly that of course -- this leads to transients in boundary
* conditions. In specific, the case where the group was idle and we start the
* one task. It takes time for our CPU's grq->avg.load_avg to build up,
* yielding bad latency etc..
*
* Now, in that special case (1) reduces to:
*
* tg->weight * grq->load.weight
* ge->load.weight = ----------------------------- = tg>weight (4)
* grp->load.weight
*
* That is, the sum collapses because all other CPUs are idle; the UP scenario.
*
* So what we do is modify our approximation (3) to approach (4) in the (near)
* UP case, like:
*
* ge->load.weight =
*
* tg->weight * grq->load.weight
* --------------------------------------------------- (5)
* tg->load_avg - grq->avg.load_avg + grq->load.weight
*
*
* 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
* overestimates the ge->load.weight and therefore:
*
* \Sum ge->load.weight >= tg->weight
*
* hence icky!
*/
static long calc_cfs_shares(struct cfs_rq *cfs_rq)
{
long tg_weight, tg_shares, load, shares;