Re: [PATCH RFC v1 1/2] vhost: option to fetch descriptors through an independent struct

[Jason Wang <jasowang@redhat.com>]

On 2019/10/16 上午4:20, Michael S. Tsirkin wrote:
> On Mon, Oct 14, 2019 at 09:43:25AM +0800, Jason Wang wrote:
>> On 2019/10/13 上午4:27, Michael S. Tsirkin wrote:
>>> On Sat, Oct 12, 2019 at 03:28:49PM +0800, Jason Wang wrote:
>>>> On 2019/10/11 下午9:45, Michael S. Tsirkin wrote:
>>>>> The idea is to support multiple ring formats by converting
>>>>> to a format-independent array of descriptors.
>>>>>
>>>>> This costs extra cycles, but we gain in ability
>>>>> to fetch a batch of descriptors in one go, which
>>>>> is good for code cache locality.
>>>>>
>>>>> To simplify benchmarking, I kept the old code
>>>>> around so one can switch back and forth by
>>>>> writing into a module parameter.
>>>>> This will go away in the final submission.
>>>>>
>>>>> This patch causes a minor performance degradation,
>>>>> it's been kept as simple as possible for ease of review.
>>>>> Next patch gets us back the performance by adding batching.
>>>>>
>>>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>>>>> ---
>>>>>     drivers/vhost/test.c  |  17 ++-
>>>>>     drivers/vhost/vhost.c | 299 +++++++++++++++++++++++++++++++++++++++++-
>>>>>     drivers/vhost/vhost.h |  16 +++
>>>>>     3 files changed, 327 insertions(+), 5 deletions(-)
>>>>>
>>>>> diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
>>>>> index 056308008288..39a018a7af2d 100644
>>>>> --- a/drivers/vhost/test.c
>>>>> +++ b/drivers/vhost/test.c
>>>>> @@ -18,6 +18,9 @@
>>>>>     #include "test.h"
>>>>>     #include "vhost.h"
>>>>> +static int newcode = 0;
>>>>> +module_param(newcode, int, 0644);
>>>>> +
>>>>>     /* Max number of bytes transferred before requeueing the job.
>>>>>      * Using this limit prevents one virtqueue from starving others. */
>>>>>     #define VHOST_TEST_WEIGHT 0x80000
>>>>> @@ -58,10 +61,16 @@ static void handle_vq(struct vhost_test *n)
>>>>>     	vhost_disable_notify(&n->dev, vq);
>>>>>     	for (;;) {
>>>>> -		head = vhost_get_vq_desc(vq, vq->iov,
>>>>> -					 ARRAY_SIZE(vq->iov),
>>>>> -					 &out, &in,
>>>>> -					 NULL, NULL);
>>>>> +		if (newcode)
>>>>> +			head = vhost_get_vq_desc_batch(vq, vq->iov,
>>>>> +						       ARRAY_SIZE(vq->iov),
>>>>> +						       &out, &in,
>>>>> +						       NULL, NULL);
>>>>> +		else
>>>>> +			head = vhost_get_vq_desc(vq, vq->iov,
>>>>> +						 ARRAY_SIZE(vq->iov),
>>>>> +						 &out, &in,
>>>>> +						 NULL, NULL);
>>>>>     		/* On error, stop handling until the next kick. */
>>>>>     		if (unlikely(head < 0))
>>>>>     			break;
>>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>>>> index 36ca2cf419bf..36661d6cb51f 100644
>>>>> --- a/drivers/vhost/vhost.c
>>>>> +++ b/drivers/vhost/vhost.c
>>>>> @@ -301,6 +301,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
>>>>>     			   struct vhost_virtqueue *vq)
>>>>>     {
>>>>>     	vq->num = 1;
>>>>> +	vq->ndescs = 0;
>>>>>     	vq->desc = NULL;
>>>>>     	vq->avail = NULL;
>>>>>     	vq->used = NULL;
>>>>> @@ -369,6 +370,9 @@ static int vhost_worker(void *data)
>>>>>     static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
>>>>>     {
>>>>> +	kfree(vq->descs);
>>>>> +	vq->descs = NULL;
>>>>> +	vq->max_descs = 0;
>>>>>     	kfree(vq->indirect);
>>>>>     	vq->indirect = NULL;
>>>>>     	kfree(vq->log);
>>>>> @@ -385,6 +389,10 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
>>>>>     	for (i = 0; i < dev->nvqs; ++i) {
>>>>>     		vq = dev->vqs[i];
>>>>> +		vq->max_descs = dev->iov_limit;
>>>>> +		vq->descs = kmalloc_array(vq->max_descs,
>>>>> +					  sizeof(*vq->descs),
>>>>> +					  GFP_KERNEL);
>>>> Is iov_limit too much here? It can obviously increase the footprint. I guess
>>>> the batching can only be done for descriptor without indirect or next set.
>>>> Then we may batch 16 or 64.
>>>>
>>>> Thanks
>>> Yes, next patch only batches up to 64.  But we do need iov_limit because
>>> guest can pass a long chain of scatter/gather.
>>> We already have iovecs in a huge array so this does not look like
>>> a big deal. If we ever teach the code to avoid the huge
>>> iov arrays by handling huge s/g lists piece by piece,
>>> we can make the desc array smaller at the same point.
>>>
>> Another possible issue, if we try to batch descriptor chain when we've
>> already batched some descriptors, we may reach the limit then some of the
>> descriptors might need re-read.
>>
>> Or we may need circular index (head, tail) in this case?
>>
>> Thanks
> We never supported more than IOV_MAX descriptors.
> And we don't batch more than iov_limit - IOV_MAX.


Ok, but what happens when we've already batched 63 descriptors then come 
a 3 descriptor chain?

And it looks to me we need forget the cached descriptor during 
set_vring_base()

Thanks


>
> so buffer never overflows.
>