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SubjectRe: [PATCH v2 0/4] implement vcpu preempted check


On 2016年07月06日 14:52, Peter Zijlstra wrote:
> On Tue, Jun 28, 2016 at 10:43:07AM -0400, Pan Xinhui wrote:
>> change fomr v1:
>> a simplier definition of default vcpu_is_preempted
>> skip mahcine type check on ppc, and add config. remove dedicated macro.
>> add one patch to drop overload of rwsem_spin_on_owner and mutex_spin_on_owner.
>> add more comments
>> thanks boqun and Peter's suggestion.
>>
>> This patch set aims to fix lock holder preemption issues.
>>
>> test-case:
>> perf record -a perf bench sched messaging -g 400 -p && perf report
>>
>> 18.09% sched-messaging [kernel.vmlinux] [k] osq_lock
>> 12.28% sched-messaging [kernel.vmlinux] [k] rwsem_spin_on_owner
>> 5.27% sched-messaging [kernel.vmlinux] [k] mutex_unlock
>> 3.89% sched-messaging [kernel.vmlinux] [k] wait_consider_task
>> 3.64% sched-messaging [kernel.vmlinux] [k] _raw_write_lock_irq
>> 3.41% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner.is
>> 2.49% sched-messaging [kernel.vmlinux] [k] system_call
>>
>> We introduce interface bool vcpu_is_preempted(int cpu) and use it in some spin
>> loops of osq_lock, rwsem_spin_on_owner and mutex_spin_on_owner.
>> These spin_on_onwer variant also cause rcu stall before we apply this patch set
>>
>
> Paolo, could you help out with an (x86) KVM interface for this?
>
> Waiman, could you see if you can utilize this to get rid of the
> SPIN_THRESHOLD in qspinlock_paravirt?
>
hmm. maybe something like below. wait_node can go into pv_wait() earlier as soon as the prev cpu is preempted.
but for the wait_head, as qspinlock does not record the lock_holder correctly(thanks to lock stealing), vcpu preemption check might get wrong results.

Waiman, I have used one hash table to keep the lock holder in my ppc implementation patch. I think we could do something similar in generic code?

diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index 74c4a86..40560e8 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -312,7 +312,8 @@ pv_wait_early(struct pv_node *prev, int loop)
if ((loop & PV_PREV_CHECK_MASK) != 0)
return false;

- return READ_ONCE(prev->state) != vcpu_running;
+ return READ_ONCE(prev->state) != vcpu_running ||
+ vcpu_is_preempted(prev->cpu);
}

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