netdev - Re: [PATCH RFC v2] ptr_ring: add ptr_ring

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Date:   Wed, 10 May 2017 10:01:23 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC v2] ptr_ring: add ptr_ring_unconsume



On 2017年05月09日 21:26, Michael S. Tsirkin wrote:
> On Wed, Apr 26, 2017 at 05:09:42PM +0800, Jason Wang wrote:
>>
>> On 2017年04月25日 00:01, Michael S. Tsirkin wrote:
>>> Applications that consume a batch of entries in one go
>>> can benefit from ability to return some of them back
>>> into the ring.
>>>
>>> Add an API for that - assuming there's space. If there's no space
>>> naturally can't do this and have to drop entries, but this implies ring
>>> is full so we'd likely drop some anyway.
>>>
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>>> ---
>>>
>>> Jason, if you add this and unconsume the outstanding packets
>>> on backend disconnect, vhost close and reset, I think
>>> we should apply your patch even if we don't yet know 100%
>>> why it helps.
>>>
>>> changes from v1:
>>> - fix up coding style issues reported by Sergei Shtylyov
>>>
>>>
>>>    include/linux/ptr_ring.h | 56 ++++++++++++++++++++++++++++++++++++++++++++++++
>>>    1 file changed, 56 insertions(+)
>>>
>>> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
>>> index 783e7f5..902afc2 100644
>>> --- a/include/linux/ptr_ring.h
>>> +++ b/include/linux/ptr_ring.h
>>> @@ -457,6 +457,62 @@ static inline int ptr_ring_init(struct ptr_ring *r, int size, gfp_t gfp)
>>>    	return 0;
>>>    }
>>> +/*
>>> + * Return entries into ring. Destroy entries that don't fit.
>>> + *
>>> + * Note: this is expected to be a rare slow path operation.
>>> + *
>>> + * Note: producer lock is nested within consumer lock, so if you
>>> + * resize you must make sure all uses nest correctly.
>>> + * In particular if you consume ring in interrupt or BH context, you must
>>> + * disable interrupts/BH when doing so.
>>> + */
>>> +static inline void ptr_ring_unconsume(struct ptr_ring *r, void **batch, int n,
>>> +				      void (*destroy)(void *))
>>> +{
>>> +	unsigned long flags;
>>> +	int head;
>>> +
>>> +	spin_lock_irqsave(&r->consumer_lock, flags);
>>> +	spin_lock(&r->producer_lock);
>>> +
>>> +	if (!r->size)
>>> +		goto done;
>>> +
>>> +	/*
>>> +	 * Clean out buffered entries (for simplicity). This way following code
>>> +	 * can test entries for NULL and if not assume they are valid.
>>> +	 */
>>> +	head = r->consumer_head - 1;
>>> +	while (likely(head >= r->consumer_tail))
>>> +		r->queue[head--] = NULL;
>>> +	r->consumer_tail = r->consumer_head;
>>> +
>>> +	/*
>>> +	 * Go over entries in batch, start moving head back and copy entries.
>>> +	 * Stop when we run into previously unconsumed entries.
>>> +	 */
>>> +	while (n--) {
>>> +		head = r->consumer_head - 1;
>>> +		if (head < 0)
>>> +			head = r->size - 1;
>>> +		if (r->queue[head]) {
>>> +			/* This batch entry will have to be destroyed. */
>>> +			++n;
>>> +			goto done;
>>> +		}
>>> +		r->queue[head] = batch[n];
>>> +		r->consumer_tail = r->consumer_head = head;
>> Looks like something wrong here (bad page state reported), uncomment the
>> above while() solving the issue. But after staring it for a while I didn't
>> find anything interesting, maybe you have some idea on this?
>>
>> Thanks
>>
>>
>>> +	}
>>> +
>>> +done:
>>> +	/* Destroy all entries left in the batch. */
>>> +	while (n--)
>>> +		destroy(batch[n]);
>>> +	spin_unlock(&r->producer_lock);
>>> +	spin_unlock_irqrestore(&r->consumer_lock, flags);
>>> +}
>>> +
>>>    static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
>>>    					   int size, gfp_t gfp,
>>>    					   void (*destroy)(void *))
> What's our plan here? I can't delay pull request much longer.
>

I'm waiting for net-next to be opened (since the series touches tun/tap).

Let me post a new version soon.

Thanks