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authorChristian Borntraeger <borntraeger@de.ibm.com>2008-05-08 15:20:38 +0200
committerRusty Russell <rusty@rustcorp.com.au>2008-05-23 13:09:34 +1000
commit3401a61e16a5b852d4e353c8850c857105a67a9c (patch)
tree5e05731a790c48512c7fe41c5fc83b0fee6081e3 /kernel
parent4d2e7d0d77e4e1e8a21cc990c607985fdba20e66 (diff)
stop_machine: make stop_machine_run more virtualization friendly
On kvm I have seen some rare hangs in stop_machine when I used more guest cpus than hosts cpus. e.g. 32 guest cpus on 1 host cpu triggered the hang quite often. I could also reproduce the problem on a 4 way z/VM host with a 64 way guest. It turned out that the guest was consuming all available cpus mostly for spinning on scheduler locks like rq->lock. This is expected as the threads are calling yield all the time. The problem is now, that the host scheduling decisings together with the guest scheduling decisions and spinlocks not being fair managed to create an interesting scenario similar to a live lock. (Sometimes the hang resolved itself after some minutes) Changing stop_machine to yield the cpu to the hypervisor when yielding inside the guest fixed the problem for me. While I am not completely happy with this patch, I think it causes no harm and it really improves the situation for me. I used cpu_relax for yielding to the hypervisor, does that work on all architectures? p.s.: If you want to reproduce the problem, cpu hotplug and kprobes use stop_machine_run and both triggered the problem after some retries. Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> CC: Ingo Molnar <mingo@elte.hu> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/stop_machine.c7
1 files changed, 4 insertions, 3 deletions
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 0101aeef7ed7..b7350bbfb076 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -62,8 +62,7 @@ static int stopmachine(void *cpu)
* help our sisters onto their CPUs. */
if (!prepared && !irqs_disabled)
yield();
- else
- cpu_relax();
+ cpu_relax();
}
/* Ack: we are exiting. */
@@ -106,8 +105,10 @@ static int stop_machine(void)
}
/* Wait for them all to come to life. */
- while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
+ while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads) {
yield();
+ cpu_relax();
+ }
/* If some failed, kill them all. */
if (ret < 0) {

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