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Date:   Sun, 24 Oct 2021 18:29:59 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     <linux-kernel@...r.kernel.org>, Jason Wang <jasowang@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        <virtualization@...ts.linux-foundation.org>,
        <linux-block@...r.kernel.org>
Subject: Re: [PATCH] virtio_blk: allow 0 as num_request_queues


On 10/24/2021 6:11 PM, Michael S. Tsirkin wrote:
> On Sun, Oct 24, 2021 at 05:19:33PM +0300, Max Gurtovoy wrote:
>> On 10/24/2021 4:54 PM, Michael S. Tsirkin wrote:
>>> The default value is 0 meaning "no limit". However if 0
>>> is specified on the command line it is instead silently
>>> converted to 1. Further, the value is already validated
>>> at point of use, there's no point in duplicating code
>>> validating the value when it is set.
>>>
>>> Simplify the code while making the behaviour more consistent
>>> by using plain module_param.
>>>
>>> Fixes: 1a662cf6cb9a ("virtio-blk: add num_request_queues module parameter")
>>> Cc: Max Gurtovoy <mgurtovoy@...dia.com>
>>> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
>>> ---
>>>    drivers/block/virtio_blk.c | 14 +-------------
>>>    1 file changed, 1 insertion(+), 13 deletions(-)
>>>
>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>>> index 6318134aab76..c336d9bb9105 100644
>>> --- a/drivers/block/virtio_blk.c
>>> +++ b/drivers/block/virtio_blk.c
>>> @@ -30,20 +30,8 @@
>>>    #define VIRTIO_BLK_INLINE_SG_CNT	2
>>>    #endif
>>> -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);
>>> -}
>>> -
>>> -static const struct kernel_param_ops queue_count_ops = {
>>> -	.set = virtblk_queue_count_set,
>>> -	.get = param_get_uint,
>>> -};
>>> -
>>>    static unsigned int num_request_queues;
>>> -module_param_cb(num_request_queues, &queue_count_ops, &num_request_queues,
>>> -		0644);
>>> +module_param(num_request_queues, uint, 0644);
>> Not the best solution.
>>
>> You can set the param to 1024 but in practice nr_cpu_ids can be 32 for
>> example.
> Well your patch does make it possible to know what nr_cpu_ids is.
> But does it matter? The actual number of queues is still capped
> by the num_queues of the device. And I'm concerned that some
> userspace comes to depend on reading back nr_cpu_ids
> from there, which will cement this as part of ABI instead of
> being an implementation detail.
> Exposing the actual number of queues in sysfs might make more sense ...
>
> Generally you suggested embedded as a use-case, and I don't really
> see lots of embedded userspace poking at this parameter in sysfs.
>
> What I'd like to see, and attempted to achieve here:
> - avoid code duplication
> - consistency: some way to specify the parameter but still make it have the default value
>
> Better suggestions are welcome.

Just change return param_set_uint_minmax(val, kp, 1, nr_cpu_ids);

to

return param_set_uint_minmax(val, kp, *0*, nr_cpu_ids);

The real amount can be exposed by common sysfs.

We'll extend virtio_driver to have a new callback to return this value. 
If callback doesn't exist - return -1 (unknown value).

>
>>>    MODULE_PARM_DESC(num_request_queues,
>>>    		 "Limit the number of request queues to use for blk device. "
>>>    		 "0 for no limit. "

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