Re: [PATCH v3 2/4] virtio_test: use random length scatterlists to test descriptor chain

[Guo Zhi <qtxuning1999@sjtu.edu.cn>]

On 2022/7/11 23:35, Eugenio Perez Martin wrote:
> On Sat, Jul 9, 2022 at 4:28 AM Guo Zhi <qtxuning1999@sjtu.edu.cn> wrote:
>> Prior implementation only use one descriptor for each io event, which
>> does't test code of descriptor chain. More importantly, one descriptor
>> will not use indirect feature even indirect feature is specified. Use
>> random length scatterlists here to test descriptor chain.
>>
>> Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
>> ---
>> v3:
>> - drop fda270fcd virtio_test: move magic number in code as defined constant
>> ---
>>   tools/virtio/virtio_test.c | 11 ++++++++---
>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
>> index 95f78b311..1408a4a20 100644
>> --- a/tools/virtio/virtio_test.c
>> +++ b/tools/virtio/virtio_test.c
>> @@ -20,6 +20,7 @@
>>   #include "../../drivers/vhost/test.h"
>>
>>   #define RANDOM_BATCH -1
>> +#define MAX_SG_FRAGS 8UL
>>
>>   /* Unused */
>>   void *__kmalloc_fake, *__kfree_ignore_start, *__kfree_ignore_end;
>> @@ -169,7 +170,8 @@ static void wait_for_interrupt(struct vdev_info *dev)
>>   static void run_test(struct vdev_info *dev, struct vq_info *vq,
>>                       bool delayed, int batch, int reset_n, int bufs)
>>   {
>> -       struct scatterlist sl;
>> +       struct scatterlist sg[MAX_SG_FRAGS];
>> +       int sg_size = 0;
>>          long started = 0, completed = 0, next_reset = reset_n;
>>          long completed_before, started_before;
>>          int r, test = 1;
>> @@ -194,8 +196,11 @@ static void run_test(struct vdev_info *dev, struct vq_info *vq,
>>
>>                          while (started < bufs &&
>>                                 (started - completed) < batch) {
>> -                               sg_init_one(&sl, dev->buf, dev->buf_size);
>> -                               r = virtqueue_add_outbuf(vq->vq, &sl, 1,
>> +                               sg_size = random() % (MAX_SG_FRAGS - 1) + 1;
> I'm wondering if it would be simpler to reuse batch randomness here,
> and make sg_size = MIN(started - completed, MAX_SG_FRAGS). Vhost test
> should go faster because the longer chains, and I guess we should hit
> a good range of chain lengths with the batch tail anyway.
>
> Thanks!

IMHO, if we reuse batch randomness here, the random length of 
scatterlist only appears when --batch=random selected.

Otherwise, when we have to specify the batch size(eg, 256), the 
scatterlist( as well as the descriptor chain len) will not be randomed. 
So I propose decouple the randomness of batch size and descriptor chain 
length.

If we have to achive better performance for vhost_test, just enlarge the 
MAX_SG_FRAGS is ok.

>> +                               sg_init_table(sg, sg_size);
>> +                               for (int i = 0; i < sg_size; ++i)
>> +                                       sg_set_buf(&sg[i], dev->buf + i, 0x1);
>> +                               r = virtqueue_add_outbuf(vq->vq, sg, sg_size,
>>                                                           dev->buf + started,
>>                                                           GFP_ATOMIC);
>>                                  if (unlikely(r != 0)) {
>> --
>> 2.17.1
>>