On 06/20/2012 02:21 AM, Rik van Riel wrote:
On Wed, 20 Jun 2012 01:50:50 +0530
Raghavendra K T<raghavendra.kt@xxxxxxxxxxxxxxxxxx> wrote:
In ple handler code, last_boosted_vcpu (lbv) variable is
serving as reference point to start when we enter.
Also statistical analysis (below) is showing lbv is not very well
distributed with current approach.
You are the second person to spot this bug today (yes, today).
Oh! really interesting.
Due to time zones, the first person has not had a chance yet to
test the patch below, which might fix the issue...
May be his timezone also falls near to mine. I am also pretty late
now. :)
Please let me know how it goes.
Yes, have got result today, too tired to summarize. got better
performance result too. will come back again tomorrow morning.
have to post, randomized start point patch also, which I discussed to
know the opinion.
====8<====
If last_boosted_vcpu == 0, then we fall through all test cases and
may end up with all VCPUs pouncing on vcpu 0. With a large enough
guest, this can result in enormous runqueue lock contention, which
can prevent vcpu0 from running, leading to a livelock.
Changing< to<= makes sure we properly handle that case.
Analysis shows distribution is more flatten now than before.
Here are the snapshots:
snapshot1
PLE handler yield stat :
66447 132222 75510 65875 121298 92543 111267 79523
118134 105366 116441 114195 107493 66666 86779 87733
84415 105778 94210 73197 55626 93036 112959 92035
95742 78558 72190 101719 94667 108593 63832 81580
PLE handler start stat :
334301 687807 384077 344917 504917 343988 439810 371389
466908 415509 394304 484276 376510 292821 370478 363727
366989 423441 392949 309706 292115 437900 413763 346135
364181 323031 348405 399593 336714 373995 302301 347383
snapshot2
PLE handler yield stat :
320547 267528 264316 164213 249246 182014 246468 225386
277179 310659 349767 310281 238680 187645 225791 266290
216202 316974 231077 216586 151679 356863 266031 213047
306229 182629 229334 241204 275975 265086 282218 242207
PLE handler start stat :
1335370 1378184 1252001 925414 1196973 951298 1219835 1108788
1265427 1290362 1308553 1271066 1107575 980036 1077210 1278611
1110779 1365130 1151200 1049859 937159 1577830 1209099 993391
1173766 987307 1144775 1102960 1100082 1177134 1207862 1119551
Signed-off-by: Rik van Riel<riel@xxxxxxxxxx>
---
virt/kvm/kvm_main.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7e14068..1da542b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1586,7 +1586,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
*/
for (pass = 0; pass< 2&& !yielded; pass++) {
kvm_for_each_vcpu(i, vcpu, kvm) {
- if (!pass&& i< last_boosted_vcpu) {
+ if (!pass&& i<= last_boosted_vcpu) {
Hmmm true, great catch. it was partial towards zero earlier.
i = last_boosted_vcpu;
continue;
} else if (pass&& i> last_boosted_vcpu)
--
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