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Date: Fri, 18 Nov 2011 15:28:23 -0800
From: tip-bot for Peter Zijlstra <a.p.zijlstra@...llo.nl>
To: linux-tip-commits@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...hat.com,
a.p.zijlstra@...llo.nl, tglx@...utronix.de, mingo@...e.hu
Subject: [tip:sched/core] sched: Add a comment to effective_load() since it's a pain
Commit-ID: cf5f0acf3935c91379e709a71ecf68805d366659
Gitweb: http://git.kernel.org/tip/cf5f0acf3935c91379e709a71ecf68805d366659
Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
AuthorDate: Thu, 13 Oct 2011 16:52:28 +0200
Committer: Ingo Molnar <mingo@...e.hu>
CommitDate: Mon, 14 Nov 2011 12:50:32 +0100
sched: Add a comment to effective_load() since it's a pain
Every time I have to stare at this function I need to completely
reverse engineer its workings, about time I write a comment
explaining the thing.
Collected bits and pieces from previous changelogs, mostly:
4be9daaa1b33701f011f4117f22dc1e45a3e6e34
83378269a5fad98f562ebc0f09c349575e6cbfe1
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/r/1318518057.27731.2.camel@twins
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
kernel/sched_fair.c | 113 ++++++++++++++++++++++++++++++++++++++++++--------
1 files changed, 95 insertions(+), 18 deletions(-)
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 5c9e679..aba20f4 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -772,19 +772,32 @@ static void update_cfs_load(struct cfs_rq *cfs_rq, int global_update)
list_del_leaf_cfs_rq(cfs_rq);
}
+static inline long calc_tg_weight(struct task_group *tg, struct cfs_rq *cfs_rq)
+{
+ long tg_weight;
+
+ /*
+ * Use this CPU's actual weight instead of the last load_contribution
+ * to gain a more accurate current total weight. See
+ * update_cfs_rq_load_contribution().
+ */
+ tg_weight = atomic_read(&tg->load_weight);
+ tg_weight -= cfs_rq->load_contribution;
+ tg_weight += cfs_rq->load.weight;
+
+ return tg_weight;
+}
+
static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
{
- long load_weight, load, shares;
+ long tg_weight, load, shares;
+ tg_weight = calc_tg_weight(tg, cfs_rq);
load = cfs_rq->load.weight;
- load_weight = atomic_read(&tg->load_weight);
- load_weight += load;
- load_weight -= cfs_rq->load_contribution;
-
shares = (tg->shares * load);
- if (load_weight)
- shares /= load_weight;
+ if (tg_weight)
+ shares /= tg_weight;
if (shares < MIN_SHARES)
shares = MIN_SHARES;
@@ -2036,36 +2049,100 @@ static void task_waking_fair(struct task_struct *p)
* Adding load to a group doesn't make a group heavier, but can cause movement
* of group shares between cpus. Assuming the shares were perfectly aligned one
* can calculate the shift in shares.
+ *
+ * Calculate the effective load difference if @wl is added (subtracted) to @tg
+ * on this @cpu and results in a total addition (subtraction) of @wg to the
+ * total group weight.
+ *
+ * Given a runqueue weight distribution (rw_i) we can compute a shares
+ * distribution (s_i) using:
+ *
+ * s_i = rw_i / \Sum rw_j (1)
+ *
+ * Suppose we have 4 CPUs and our @tg is a direct child of the root group and
+ * has 7 equal weight tasks, distributed as below (rw_i), with the resulting
+ * shares distribution (s_i):
+ *
+ * rw_i = { 2, 4, 1, 0 }
+ * s_i = { 2/7, 4/7, 1/7, 0 }
+ *
+ * As per wake_affine() we're interested in the load of two CPUs (the CPU the
+ * task used to run on and the CPU the waker is running on), we need to
+ * compute the effect of waking a task on either CPU and, in case of a sync
+ * wakeup, compute the effect of the current task going to sleep.
+ *
+ * So for a change of @wl to the local @cpu with an overall group weight change
+ * of @wl we can compute the new shares distribution (s'_i) using:
+ *
+ * s'_i = (rw_i + @wl) / (@wg + \Sum rw_j) (2)
+ *
+ * Suppose we're interested in CPUs 0 and 1, and want to compute the load
+ * differences in waking a task to CPU 0. The additional task changes the
+ * weight and shares distributions like:
+ *
+ * rw'_i = { 3, 4, 1, 0 }
+ * s'_i = { 3/8, 4/8, 1/8, 0 }
+ *
+ * We can then compute the difference in effective weight by using:
+ *
+ * dw_i = S * (s'_i - s_i) (3)
+ *
+ * Where 'S' is the group weight as seen by its parent.
+ *
+ * Therefore the effective change in loads on CPU 0 would be 5/56 (3/8 - 2/7)
+ * times the weight of the group. The effect on CPU 1 would be -4/56 (4/8 -
+ * 4/7) times the weight of the group.
*/
static long effective_load(struct task_group *tg, int cpu, long wl, long wg)
{
struct sched_entity *se = tg->se[cpu];
- if (!tg->parent)
+ if (!tg->parent) /* the trivial, non-cgroup case */
return wl;
for_each_sched_entity(se) {
- long lw, w;
+ long w, W;
tg = se->my_q->tg;
- w = se->my_q->load.weight;
- /* use this cpu's instantaneous contribution */
- lw = atomic_read(&tg->load_weight);
- lw -= se->my_q->load_contribution;
- lw += w + wg;
+ /*
+ * W = @wg + \Sum rw_j
+ */
+ W = wg + calc_tg_weight(tg, se->my_q);
- wl += w;
+ /*
+ * w = rw_i + @wl
+ */
+ w = se->my_q->load.weight + wl;
- if (lw > 0 && wl < lw)
- wl = (wl * tg->shares) / lw;
+ /*
+ * wl = S * s'_i; see (2)
+ */
+ if (W > 0 && w < W)
+ wl = (w * tg->shares) / W;
else
wl = tg->shares;
- /* zero point is MIN_SHARES */
+ /*
+ * Per the above, wl is the new se->load.weight value; since
+ * those are clipped to [MIN_SHARES, ...) do so now. See
+ * calc_cfs_shares().
+ */
if (wl < MIN_SHARES)
wl = MIN_SHARES;
+
+ /*
+ * wl = dw_i = S * (s'_i - s_i); see (3)
+ */
wl -= se->load.weight;
+
+ /*
+ * Recursively apply this logic to all parent groups to compute
+ * the final effective load change on the root group. Since
+ * only the @tg group gets extra weight, all parent groups can
+ * only redistribute existing shares. @wl is the shift in shares
+ * resulting from this level per the above.
+ */
wg = 0;
}
--
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