Re: [PATCH v2 1/1] virtio-blk: add num_io_queues module parameter

On Mon, Sep 06, 2021 at 02:59:40PM +0300, Max Gurtovoy wrote:

On 9/6/2021 2:20 PM, Michael S. Tsirkin wrote:

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On Mon, Sep 06, 2021 at 01:31:32AM +0300, Max Gurtovoy wrote:

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On 9/5/2021 7:02 PM, Michael S. Tsirkin wrote:

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On Thu, Sep 02, 2021 at 02:45:52PM +0100, Stefan Hajnoczi wrote:

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On Tue, Aug 31, 2021 at 04:50:35PM +0300, Max Gurtovoy wrote:

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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@nvidia.com>
---

changes from v1:
   - use param_set_uint_minmax (from Christoph)
   - added "Should > 0" to module description

Note: This commit apply on top of Jens's branch for-5.15/drivers
---
   drivers/block/virtio_blk.c | 20 +++++++++++++++++++-
   1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4b49df2dfd23..9332fc4e9b31 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c

@@ -24,6 +24,22 @@
   /* 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)
+{
+	return param_set_uint_minmax(val, kp, 1, nr_cpu_ids);
+}

Hmm which tree is this for?

I've mentioned in the note that it apply on branch for-5.15/drivers. But now
you can apply it on linus/master as well.

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+
+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. Should > 0");

better:

+MODULE_PARM_DESC(num_io_request_queues,
+                "Limit number of IO request virt queues to use for each device. 0 for now limit");

You proposed it and I replied on it bellow.

Can't say I understand 100% what you are saying. Want to send
a description that does make sense to you but
also reflects reality? 0 is the default so
"should > 0" besides being ungrammatical does not seem t"
reflect what it does ...

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+
   static int major;
   static DEFINE_IDA(vd_index_ida);

@@ -501,7 +517,9 @@ 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);

If you respin, please consider calling them request queues. That's the
terminology from the VIRTIO spec and it's nice to keep it consistent.
But the purpose of num_io_queues is clear, so:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

I did this:
+static unsigned int num_io_request_queues;
+module_param_cb(num_io_request_queues, &queue_count_ops, &num_io_request_queues, 0644);
+MODULE_PARM_DESC(num_io_request_queues,
+                "Limit number of IO request virt queues to use for each device. 0 for now limit");

The parameter is writable and can be changed and then new devices might be
probed with new value.

It can't be zero in the code. we can change param_set_uint_minmax args and
say that 0 says nr_cpus.

I'm ok with the renaming but I prefer to stick to the description we gave in
V3 of this patch (and maybe enable value of 0 as mentioned above).

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