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SubjectRe: [PATCH] kvm: add halt_poll_ns module parameter


On 09/02/2015 16:21, Radim Krčmář wrote:
> 2015-02-06 13:48+0100, Paolo Bonzini:
> [...]
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>
> Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
>
> Noticed changes since RFC:
> - polling is used in more situations
> - new tracepoint
> - module parameter in nanoseconds
> - properly handled time
> - no polling with overcommit

Yup, pretty much what came in from Marcelo and David.

>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> @@ -148,6 +148,7 @@ struct kvm_vm_stat {
>> };
>>
>> struct kvm_vcpu_stat {
>> + u32 halt_successful_poll;
>> u32 halt_wakeup;
>> };
>
> We don't expose it in arch/arm/kvm/guest.c,
> struct kvm_stats_debugfs_item debugfs_entries[] = {
> { NULL }
> };

Yes. Too late for 3.20.

>> +TRACE_EVENT(kvm_vcpu_wakeup,
>> + TP_PROTO(__u64 ns, bool waited),
>
> (__u64 is preferred here?)

Preferred to what?

>> @@ -1813,29 +1816,60 @@ void mark_page_dirty(struct kvm *kvm, gfn_t gfn)
>> void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> {
>> + ktime_t start, cur;
>> DEFINE_WAIT(wait);
>> + bool waited = false;
>> +
>> + start = cur = ktime_get();
>> + if (halt_poll_ns) {
>> + ktime_t stop = ktime_add_ns(ktime_get(), halt_poll_ns);
>> + do {
>> + /*
>> + * This sets KVM_REQ_UNHALT if an interrupt
>> + * arrives.
>> + */
>> + if (kvm_vcpu_check_block(vcpu) < 0) {
>> + ++vcpu->stat.halt_successful_poll;
>> + goto out;
>> + }
>> + cur = ktime_get();
>> + } while (single_task_running() && ktime_before(cur, stop));
>
> After reading a bunch of code, I'm still not sure ...
> - need_resched() can't be true when single_task_running()?
> (I think it could happen -- balancing comes into mind.)

Single_task_running is per-CPU; for a task to relinquish control to
another task, you first need to have multiple tasks running. In other
words, I think it cannot.

> - is it ok to ignore need_resched() when single_task_running()?
> (Most likely not.)

Paolo


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