linux-kernel - Re: questions on calc_delta

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Date:	Fri, 02 May 2008 14:30:55 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Joel Schopp <jschopp@...tin.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: questions on calc_delta_mine() in sched.c

On Fri, 2008-05-02 at 14:14 +0200, Peter Zijlstra wrote:

> And, no sadly I have no ideas on how to get rid of it ;-/

Well, not quite true - I've once thought about a small cache of recently
computed inverse values. Esp for say a ping-pong scenario you end up
re-computing basically the same values over and over again.

Maybe something like this (fully untested):

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
 kernel/sched.c |   22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

Index: linux-2.6-2/kernel/sched.c
===================================================================
--- linux-2.6-2.orig/kernel/sched.c
+++ linux-2.6-2/kernel/sched.c
@@ -537,6 +537,9 @@ struct rq {
 	unsigned long nr_load_updates;
 	u64 nr_switches;
 
+	struct load_weight[4] lw_cache;
+	int lw_cache_idx;
+
 	struct cfs_rq cfs;
 	struct rt_rq rt;
 
@@ -1438,8 +1441,19 @@ calc_delta_mine(unsigned long delta_exec
 {
 	u64 tmp;
 
-	if (unlikely(!lw->inv_weight))
-		lw->inv_weight = (WMULT_CONST-lw->weight/2) / (lw->weight+1);
+	if (!lw->inv_weight) {
+		struct rq *rq = cpu_rq(smp_processor_id());
+
+		for (i = 0; i < ARRAY_SIZE(rq->lw_cache); i++) {
+			if (rq->lw_cache[i].weight == lw->weight)
+				lw->inv_weight = rq->lw_cache[i].inv_weight;
+			goto got_inv;
+		}
+		lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2) / (lw->weight+1);
+		rq->lw_cache[rq->lw_cache_idx] = *lw;
+		rq->lw_cache_idx = (rq->lw_cache_idx + 1) % ARRAY_SIZE(rq->lw_cache);
+	}
+ got_inv:
 
 	tmp = (u64)delta_exec * weight;
 	/*
@@ -8025,7 +8039,7 @@ static void init_tg_cfs_entry(struct tas
 
 	se->my_q = cfs_rq;
 	se->load.weight = tg->shares;
-	se->load.inv_weight = div64_64(1ULL<<32, se->load.weight);
+	se->load.inv_weight = 0;
 	se->parent = parent;
 }
 #endif
@@ -8692,7 +8706,7 @@ static void __set_se_shares(struct sched
 		dequeue_entity(cfs_rq, se, 0);
 
 	se->load.weight = shares;
-	se->load.inv_weight = div64_64((1ULL<<32), shares);
+	se->load.inv_weight = 0;
 
 	if (on_rq)
 		enqueue_entity(cfs_rq, se, 0);


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