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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 <[email protected]> CC: Ingo Molnar <[email protected]> Signed-off-by: Rusty Russell <[email protected]>
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