linux-kernel - Re: [PATCH net-next 1/8] ptr_ring: introduce batch dequeuing

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Date:   Thu, 23 Mar 2017 13:33:28 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/8] ptr_ring: introduce batch dequeuing



On 2017年03月22日 21:43, Michael S. Tsirkin wrote:
> On Tue, Mar 21, 2017 at 12:04:40PM +0800, Jason Wang wrote:
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   include/linux/ptr_ring.h | 65 ++++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 65 insertions(+)
>>
>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
>> index 6c70444..4771ded 100644
>> --- a/include/linux/ptr_ring.h
>> +++ b/include/linux/ptr_ring.h
>> @@ -247,6 +247,22 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
>>   	return ptr;
>>   }
>>   
>> +static inline int __ptr_ring_consume_batched(struct ptr_ring *r,
>> +					     void **array, int n)
>> +{
>> +	void *ptr;
>> +	int i = 0;
>> +
>> +	while (i < n) {
>> +		ptr = __ptr_ring_consume(r);
>> +		if (!ptr)
>> +			break;
>> +		array[i++] = ptr;
>> +	}
>> +
>> +	return i;
>> +}
>> +
>>   /*
>>    * Note: resize (below) nests producer lock within consumer lock, so if you
>>    * call this in interrupt or BH context, you must disable interrupts/BH when
>
> This ignores the comment above that function:
>
> /* Note: callers invoking this in a loop must use a compiler barrier,
>   * for example cpu_relax().
>   */

Yes, __ptr_ring_swap_queue() ignores this too.

>
> Also - it looks like it shouldn't matter if reads are reordered but I wonder.
> Thoughts? Including some reasoning about it in commit log would be nice.

Yes, I think it doesn't matter in this case, it matters only for batched 
producing.

Thanks

>
>> @@ -297,6 +313,55 @@ static inline void *ptr_ring_consume_bh(struct ptr_ring *r)
>>   	return ptr;
>>   }
>>   
>> +static inline int ptr_ring_consume_batched(struct ptr_ring *r,
>> +					   void **array, int n)
>> +{
>> +	int ret;
>> +
>> +	spin_lock(&r->consumer_lock);
>> +	ret = __ptr_ring_consume_batched(r, array, n);
>> +	spin_unlock(&r->consumer_lock);
>> +
>> +	return ret;
>> +}
>> +
>> +static inline int ptr_ring_consume_batched_irq(struct ptr_ring *r,
>> +					       void **array, int n)
>> +{
>> +	int ret;
>> +
>> +	spin_lock_irq(&r->consumer_lock);
>> +	ret = __ptr_ring_consume_batched(r, array, n);
>> +	spin_unlock_irq(&r->consumer_lock);
>> +
>> +	return ret;
>> +}
>> +
>> +static inline int ptr_ring_consume_batched_any(struct ptr_ring *r,
>> +					       void **array, int n)
>> +{
>> +	unsigned long flags;
>> +	int ret;
>> +
>> +	spin_lock_irqsave(&r->consumer_lock, flags);
>> +	ret = __ptr_ring_consume_batched(r, array, n);
>> +	spin_unlock_irqrestore(&r->consumer_lock, flags);
>> +
>> +	return ret;
>> +}
>> +
>> +static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>> +					      void **array, int n)
>> +{
>> +	int ret;
>> +
>> +	spin_lock_bh(&r->consumer_lock);
>> +	ret = __ptr_ring_consume_batched(r, array, n);
>> +	spin_unlock_bh(&r->consumer_lock);
>> +
>> +	return ret;
>> +}
>> +
>>   /* Cast to structure type and call a function without discarding from FIFO.
>>    * Function must return a value.
>>    * Callers must take consumer_lock.
>> -- 
>> 2.7.4