sched: Avoid division by zero

Patch a5004278f0525dcb9aa43703ef77bf371ea837cd (sched: Fix
cgroup smp fairness) introduced the possibility of a
divide-by-zero because load-balancing is not synchronized
between sched_domains.

This can cause the state of cpus to change between the first
and second loop over the sched domain in tg_shares_up().

Reported-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Jes Sorensen <jes@sgi.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <1250855934.7538.30.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/sched.c b/kernel/sched.c
index 1b529ef..8f8a98e 100644 (file)
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1522,7 +1522,8 @@ static void __set_se_shares(struct sched_entity *se, unsigned long shares);
  */
 static void
 update_group_shares_cpu(struct task_group *tg, int cpu,
-                       unsigned long sd_shares, unsigned long sd_rq_weight)
+                       unsigned long sd_shares, unsigned long sd_rq_weight,
+                       unsigned long sd_eff_weight)
 {
        unsigned long rq_weight;
        unsigned long shares;
@@ -1535,13 +1536,15 @@ update_group_shares_cpu(struct task_group *tg, int cpu,
        if (!rq_weight) {
                boost = 1;
                rq_weight = NICE_0_LOAD;
+               if (sd_rq_weight == sd_eff_weight)
+                       sd_eff_weight += NICE_0_LOAD;
+               sd_rq_weight = sd_eff_weight;
        }
 
        /*
-        *           \Sum shares * rq_weight
-        * shares =  -----------------------
-        *               \Sum rq_weight
-        *
+        *             \Sum_j shares_j * rq_weight_i
+        * shares_i =  -----------------------------
+        *                  \Sum_j rq_weight_j
         */
        shares = (sd_shares * rq_weight) / sd_rq_weight;
        shares = clamp_t(unsigned long, shares, MIN_SHARES, MAX_SHARES);
@@ -1593,14 +1596,8 @@ static int tg_shares_up(struct task_group *tg, void *data)
        if (!sd->parent || !(sd->parent->flags & SD_LOAD_BALANCE))
                shares = tg->shares;
 
-       for_each_cpu(i, sched_domain_span(sd)) {
-               unsigned long sd_rq_weight = rq_weight;
-
-               if (!tg->cfs_rq[i]->rq_weight)
-                       sd_rq_weight = eff_weight;
-
-               update_group_shares_cpu(tg, i, shares, sd_rq_weight);
-       }
+       for_each_cpu(i, sched_domain_span(sd))
+               update_group_shares_cpu(tg, i, shares, rq_weight, eff_weight);
 
        return 0;
 }