On 8/31/2021 12:48 AM, Michael S. Tsirkin wrote:
On Mon, Aug 30, 2021 at 03:00:23PM +0300, Max Gurtovoy wrote:
Sometimes a user would like to control the amount of IO queues to be
created for a block device. For example, for limiting the memory
footprint of virtio-blk devices.
Signed-off-by: Max Gurtovoy <mgurtovoy@xxxxxxxxxx>
Hmm. It's already limited by # of CPUs... Why not just limit
from the hypervisor side? What's the actual use-case here?
Limiting and minimizing resource allocation is a real use case.
# of CPUs today might be 64 or 128. HW virtio-blk device might have this
amount of queues (or at least 32).
But it's a waste to use all the queues since the device may reach to max
IOPs with less queues. Multiply this by 16 or 32 devices we get a lot of
memory wasted without a real need.
It's a common configuration we do in NVMf connect command and it can
also be seen in other drivers in some variation (null_blk.submit_queues,
ib_srp.ch_count and more).
Another use case is to add some flexibility for QOS.
Also if we can set the queue depth, it's a good idea to control the
queue count as well.
If no objections, I'll take the comment from Christoph and send v2.
---
drivers/block/virtio_blk.c | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index e574fbf5e6df..77e8468e8593 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -24,6 +24,28 @@
/* The maximum number of sg elements that fit into a virtqueue */
#define VIRTIO_BLK_MAX_SG_ELEMS 32768
+static int virtblk_queue_count_set(const char *val,
+ const struct kernel_param *kp)
+{
+ unsigned int n;
+ int ret;
+
+ ret = kstrtouint(val, 10, &n);
+ if (ret != 0 || n > nr_cpu_ids)
+ return -EINVAL;
+ return param_set_uint(val, kp);
+}
+
+static const struct kernel_param_ops queue_count_ops = {
+ .set = virtblk_queue_count_set,
+ .get = param_get_uint,
+};
+
+static unsigned int num_io_queues;
+module_param_cb(num_io_queues, &queue_count_ops, &num_io_queues, 0644);
+MODULE_PARM_DESC(num_io_queues,
+ "Number of IO virt queues to use for blk device.");
+
static int major;
static DEFINE_IDA(vd_index_ida);
@@ -501,7 +523,9 @@ static int init_vq(struct virtio_blk *vblk)
if (err)
num_vqs = 1;
- num_vqs = min_t(unsigned int, nr_cpu_ids, num_vqs);
+ num_vqs = min_t(unsigned int,
+ min_not_zero(num_io_queues, nr_cpu_ids),
+ num_vqs);
vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
if (!vblk->vqs)
--
2.18.1
