thr3ads.net - Linux Virtualization - [PATCH RFC v1 1/2] vhost: option to fetch descriptors through an independent struct [Oct 2019]

If this information is useful, please help other people find it:
Share via:

Jason Wang

2019-Oct-14 01:43 UTC

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

On 2019/10/13 ??4:27, Michael S. Tsirkin wrote:> On Sat, Oct 12, 2019 at 03:28:49PM +0800, Jason Wang wrote:
>> On 2019/10/11 ??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 at 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

Michael S. Tsirkin

2019-Oct-15 20:20 UTC

head link

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

On Mon, Oct 14, 2019 at 09:43:25AM +0800, Jason Wang
wrote:> 
> On 2019/10/13 ??4:27, Michael S. Tsirkin wrote:
> > On Sat, Oct 12, 2019 at 03:28:49PM +0800, Jason Wang wrote:
> > > On 2019/10/11 ??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 at 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.

so buffer never overflows.

-- 
MST

Jason Wang

2019-Oct-16 04:38 UTC

head link

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

On 2019/10/16 ??4:20, Michael S. Tsirkin wrote:> On Mon, Oct 14, 2019 at 09:43:25AM +0800, Jason Wang wrote:
>> On 2019/10/13 ??4:27, Michael S. Tsirkin wrote:
>>> On Sat, Oct 12, 2019 at 03:28:49PM +0800, Jason Wang wrote:
>>>> On 2019/10/11 ??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 at 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.
>

Maybe Matching Threads

Search for more reasonably related threads

Linux Virtualization - Oct 2019 - [PATCH RFC v1 1/2] vhost: option to fetch descriptors through an independent struct

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

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

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

Maybe Matching Threads