linux-kernel - Re: [PATCH 2/2] locking/osq: Drop the overload of osq

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Date:	Sun, 26 Jun 2016 15:12:47 +0800
From:	panxinhui <xinhui@...ux.vnet.ibm.com>
To:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
Cc:	panxinhui <xinhui@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...hat.com, boqun.feng@...il.com
Subject: Re: [PATCH 2/2] locking/osq: Drop the overload of osq_lock()


> 在 2016年6月26日，18:41，Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com> 写道：
> 
> 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.
> 
> So lets also use neet_yield_to() to detect if we need stop the spinning
> 
> Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
> ---
> kernel/locking/osq_lock.c | 18 +++++++++++++++---
> 1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> index 05a3785..4287603 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;
> @@ -84,9 +89,10 @@ osq_wait_next(struct optimistic_spin_queue *lock,
> bool osq_lock(struct optimistic_spin_queue *lock)
> {
> 	struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
> -	struct optimistic_spin_node *prev, *next;
> +	struct optimistic_spin_node *prev, *next, *prev_old;
> 	int curr = encode_cpu(smp_processor_id());
> 	int old;
> +	unsigned int yield_count;
> 
> 	node->locked = 0;
> 	node->next = NULL;
> @@ -114,14 +120,20 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> 	 * guaranteed their existence -- this allows us to apply
> 	 * cmpxchg in an attempt to undo our queueing.
> 	 */
> -
> +	prev_old = prev;
> +	yield_count = vcpu_get_yield_count(node_cpu(prev));
> 	while (!READ_ONCE(node->locked)) {
> 		/*
> 		 * If we need to reschedule bail... so we can block.
> 		 */
> -		if (need_resched())
> +		if (need_resched() ||
> +			need_yield_to(node_cpu(prev), yield_count))
> 			goto unqueue;
> 
> +		prev = READ_ONCE(node->prev);
> +		if (prev != prev_old)
> +			yield_count = vcpu_get_yield_count(node_cpu(prev));
> +

OK, one mistake , 
should be
	if (prev != prev_old) {
		prev_old = prev;
		yield_count = vcpu_get_yield_count(node_cpu(prev));
	}

> 		cpu_relax_lowlatency();
> 	}
> 	return true;
> -- 
> 2.4.11
>