* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:
hm, found a problem: this removes the 'fuzz' from balancing, which is a
slight over-balancing to perturb CPU-bound tasks to be distributed in a
fairer manner between CPUs. So how about the patch below instead?
a good testcase for this is to start 3 CPU-bound tasks on a 2-core box:
for ((i=0; i<3; i++)); do while :; do :; done & done
with your patch applied two of the loops stick to one core, getting 50%
each - the third loop sticks to the other core, getting 100% CPU time:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3093 root 20 0 2736 528 252 R 100 0.0 0:23.81 bash
3094 root 20 0 2736 532 256 R 50 0.0 0:11.95 bash
3095 root 20 0 2736 532 256 R 50 0.0 0:11.95 bash
with no patch, or with my patch below each gets ~66% of CPU time,
long-term:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2290 mingo 20 0 2736 528 252 R 67 0.0 3:22.95 bash
2291 mingo 20 0 2736 532 256 R 67 0.0 3:18.94 bash
2292 mingo 20 0 2736 532 256 R 66 0.0 3:19.83 bash
the breakage wasnt caused by the fuzz, it was caused by the /2 - the
patch below should fix this for real.
Ingo
------------------------------>
Subject: sched: fix broken SMT/MC optimizations
From: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
On a four package system with HT - HT load balancing optimizations were
broken. For example, if two tasks end up running on two logical threads
of one of the packages, scheduler is not able to pull one of the tasks
to a completely idle package.
In this scenario, for nice-0 tasks, imbalance calculated by scheduler
will be 512 and find_busiest_queue() will return 0 (as each cpu's load
is 1024 > imbalance and has only one task running).
Similarly MC scheduler optimizations also get fixed with this patch.
[ mingo@elte.hu: restored fair balancing by increasing the fuzz and
adding it back to the power decision, without the /2
factor. ]
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
include/linux/sched.h | 2 +-
kernel/sched.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -681,7 +681,7 @@ enum cpu_idle_type {
#define SCHED_LOAD_SHIFT 10
#define SCHED_LOAD_SCALE (1L << SCHED_LOAD_SHIFT)
-#define SCHED_LOAD_SCALE_FUZZ (SCHED_LOAD_SCALE >> 1)
+#define SCHED_LOAD_SCALE_FUZZ SCHED_LOAD_SCALE
#ifdef CONFIG_SMP
#define SD_LOAD_BALANCE 1 /* Do load balancing on this domain. */
Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -2517,7 +2517,7 @@ group_next:
* a think about bumping its value to force at least one task to be
* moved
*/
- if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task/2) {
+ if (*imbalance < busiest_load_per_task) {
unsigned long tmp, pwr_now, pwr_move;
unsigned int imbn;
-
