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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>
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 2017-08-24 13:06:35 +02:00 committed by Ingo Molnar
parent 2c8e4dce79
commit 17de4ee04c
1 changed files with 44 additions and 22 deletions
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66
kernel/sched/fair.c
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@ -2860,7 +2860,7 @@ void reweight_task(struct task_struct *p, int prio)
* 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.
@ -2874,6 +2874,18 @@ void reweight_task(struct task_struct *p, int prio)
* --------------------------------------------------- (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
@ -2890,10 +2902,6 @@ static long calc_group_shares(struct cfs_rq *cfs_rq)
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);
@ -2922,32 +2930,46 @@ static long calc_group_shares(struct cfs_rq *cfs_rq)
}
/*
* 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.
*
* max(cfs_rq->avg.runnable_load_avg, cfs_rq->runnable_weight)
* shares * ------------------------------------------------------------
* max(cfs_rq->avg.load_avg, cfs_rq->load.weight)
* 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.
*
* 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 */