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Subject[PATCH] [89/275] sched: Increment cache_nice_tries only on periodic lb
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2.6.35-longterm review patch.  If anyone has any objections, please let me know.

------------------
Commit: 58b26c4c025778c09c7a1438ff185080e11b7d0a upstream
scheduler uses cache_nice_tries as an indicator to do cache_hot and
active load balance, when normal load balance fails. Currently,
this value is changed on any failed load balance attempt. That ends
up being not so nice to workloads that enter/exit idle often, as
they do more frequent new_idle balance and that pretty soon results
in cache hot tasks being pulled in.

Making the cache_nice_tries ignore failed new_idle balance seems to
make better sense. With that only the failed load balance in
periodic load balance gets accounted and the rate of accumulation
of cache_nice_tries will not depend on idle entry/exit (short
running sleep-wakeup kind of tasks). This reduces movement of
cache_hot tasks.

schedstat diff (after-before) excerpt from a workload that has
frequent and short wakeup-idle pattern (:2 in cpu col below refers
to NEWIDLE idx) This snapshot was across ~400 seconds.

Without this change:
domainstats: domain0
cpu cnt bln fld imb gain hgain nobusyq nobusyg
0:2 306487 219575 73167 110069413 44583 19070 1172 218403
1:2 292139 194853 81421 120893383 50745 21902 1259 193594
2:2 283166 174607 91359 129699642 54931 23688 1287 173320
3:2 273998 161788 93991 132757146 57122 24351 1366 160422
4:2 289851 215692 62190 83398383 36377 13680 851 214841
5:2 316312 222146 77605 117582154 49948 20281 988 221158
6:2 297172 195596 83623 122133390 52801 21301 929 194667
7:2 283391 178078 86378 126622761 55122 22239 928 177150
8:2 297655 210359 72995 110246694 45798 19777 1125 209234
9:2 297357 202011 79363 119753474 50953 22088 1089 200922
10:2 278797 178703 83180 122514385 52969 22726 1128 177575
11:2 272661 167669 86978 127342327 55857 24342 1195 166474
12:2 293039 204031 73211 110282059 47285 19651 948 203083
13:2 289502 196762 76803 114712942 49339 20547 1016 195746
14:2 264446 169609 78292 115715605 50459 21017 982 168627
15:2 260968 163660 80142 116811793 51483 21281 1064 162596

With this change:
domainstats: domain0
cpu cnt bln fld imb gain hgain nobusyq nobusyg
0:2 272347 187380 77455 105420270 24975 1 953 186427
1:2 267276 172360 86234 116242264 28087 6 1028 171332
2:2 259769 156777 93281 123243134 30555 1 1043 155734
3:2 250870 143129 97627 127370868 32026 6 1188 141941
4:2 248422 177116 64096 78261112 22202 2 757 176359
5:2 275595 180683 84950 116075022 29400 6 778 179905
6:2 262418 162609 88944 119256898 31056 4 817 161792
7:2 252204 147946 92646 122388300 32879 4 824 147122
8:2 262335 172239 81631 110477214 26599 4 864 171375
9:2 261563 164775 88016 117203621 28331 3 849 163926
10:2 243389 140949 93379 121353071 29585 2 909 140040
11:2 242795 134651 98310 124768957 30895 2 1016 133635
12:2 255234 166622 79843 104696912 26483 4 746 165876
13:2 244944 151595 83855 109808099 27787 3 801 150794
14:2 241301 140982 89935 116954383 30403 6 845 140137
15:2 232271 128564 92821 119185207 31207 4 1416 127148

Signed-off-by: Venkatesh Pallipadi <venki@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andi Kleen <ak@linux.intel.com>
LKML-Reference: <1284167957-3675-1-git-send-email-venki@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
kernel/sched_fair.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
Index: linux-2.6.35.y/kernel/sched_fair.c
===================================================================
--- linux-2.6.35.y.orig/kernel/sched_fair.c 2011-03-29 23:02:59.969300955 -0700
+++ linux-2.6.35.y/kernel/sched_fair.c 2011-03-29 23:55:04.726346241 -0700
@@ -2894,7 +2894,14 @@

if (!ld_moved) {
schedstat_inc(sd, lb_failed[idle]);
- sd->nr_balance_failed++;
+ /*
+ * Increment the failure counter only on periodic balance.
+ * We do not want newidle balance, which can be very
+ * frequent, pollute the failure counter causing
+ * excessive cache_hot migrations and active balances.
+ */
+ if (idle != CPU_NEWLY_IDLE)
+ sd->nr_balance_failed++;

if (need_active_balance(sd, sd_idle, idle)) {
raw_spin_lock_irqsave(&busiest->lock, flags);

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