diff mbox

[v6,02/11] locking/osq: Drop the overload of osq_lock()

Message ID 1477642287-24104-3-git-send-email-xinhui.pan@linux.vnet.ibm.com (mailing list archive)
State Superseded
Headers show

Commit Message

xinhui Oct. 28, 2016, 8:11 a.m. UTC 
An over-committed guest with more vCPUs than pCPUs has a heavy overload in
osq_lock().

This is because vCPU A hold the osq lock and yield out, vCPU B wait per_cpu
node->locked to be set. IOW, vCPU B wait vCPU A to run and unlock the osq
lock.

Kernel has an interface bool vcpu_is_preempted(int cpu) to see if a vCPU is
currently running or not. So break the spin loops on true condition.

test case:
perf record -a perf bench sched messaging -g 400 -p && perf report

before patch:
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

after patch:
20.68%  sched-messaging  [kernel.vmlinux]  [k] mutex_spin_on_owner
 8.45%  sched-messaging  [kernel.vmlinux]  [k] mutex_unlock
 4.12%  sched-messaging  [kernel.vmlinux]  [k] system_call
 3.01%  sched-messaging  [kernel.vmlinux]  [k] system_call_common
 2.83%  sched-messaging  [kernel.vmlinux]  [k] copypage_power7
 2.64%  sched-messaging  [kernel.vmlinux]  [k] rwsem_spin_on_owner
 2.00%  sched-messaging  [kernel.vmlinux]  [k] osq_lock

Suggested-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Juergen Gross <jgross@suse.com>
---
 kernel/locking/osq_lock.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

Comments

Davidlohr Bueso Oct. 29, 2016, 4:52 p.m. UTC | #1 
On Fri, 28 Oct 2016, Pan Xinhui wrote:
> 		/*
> 		 * If we need to reschedule bail... so we can block.
>+		 * Use vcpu_is_preempted to detech lock holder preemption issue
                                            ^^ detect
>+		 * and break. 

Could you please remove the rest of this comment? Its just noise to point out
that vcpu_is_preempted is a macro defined by arch/false. This is standard protocol
in the kernel.

Same goes for all locks you change with this.

Thanks,
Davidlohr

>                * vcpu_is_preempted is a macro defined by false if
>+		 * arch does not support vcpu preempted check,
> 		 */
>-		if (need_resched())
>+		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
> 			goto unqueue;
>
> 		cpu_relax_lowlatency();
>-- 
>2.4.11
>
xinhui Oct. 30, 2016, 2:39 p.m. UTC | #2 
在 2016/10/30 00:52, Davidlohr Bueso 写道:
> On Fri, 28 Oct 2016, Pan Xinhui wrote:
>>         /*
>>          * If we need to reschedule bail... so we can block.
>> +         * Use vcpu_is_preempted to detech lock holder preemption issue
>                                            ^^ detect
ok. thanks for poingting it out.
>> +         * and break.
>
> Could you please remove the rest of this comment? Its just noise to point out
> that vcpu_is_preempted is a macro defined by arch/false. This is standard protocol
> in the kernel.
>
fair enough.

> Same goes for all locks you change with this.
>
> Thanks,
> Davidlohr
>
>>                * vcpu_is_preempted is a macro defined by false if
>> +         * arch does not support vcpu preempted check,
>>          */
>> -        if (need_resched())
>> +        if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
>>             goto unqueue;
>>
>>         cpu_relax_lowlatency();
>> --
>> 2.4.11
>>
>
diff mbox

Patch

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a3785..39d1385 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -21,6 +21,11 @@  static inline int encode_cpu(int cpu_nr)
 	return cpu_nr + 1;
 }
 
+static inline int node_cpu(struct optimistic_spin_node *node)
+{
+	return node->cpu - 1;
+}
+
 static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
 {
 	int cpu_nr = encoded_cpu_val - 1;
@@ -118,8 +123,11 @@  bool osq_lock(struct optimistic_spin_queue *lock)
 	while (!READ_ONCE(node->locked)) {
 		/*
 		 * If we need to reschedule bail... so we can block.
+		 * Use vcpu_is_preempted to detech lock holder preemption issue
+		 * and break. vcpu_is_preempted is a macro defined by false if
+		 * arch does not support vcpu preempted check,
 		 */
-		if (need_resched())
+		if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
 			goto unqueue;
 
 		cpu_relax_lowlatency();