On 7/2/18 5:43 PM, Ming Lei wrote:
> On Tue, Jul 3, 2018 at 4:37 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>> On 7/2/18 2:28 PM, Douglas Gilbert wrote:
>>> On 2018-07-02 07:06 AM, Ming Lei wrote:
>>>> With the introduced module parameter of 'use_blk_mq', it is easy
>>>> to switch between 'blk_mq' and 'non_blk_mq' by reloading scsi_debug
>>>> module, so that we can test scsi_mq/blk_mq related regressions easily.
>>>>
>>>> Cc: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
>>>> Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
>>>> Cc: Jens Axboe <axboe@xxxxxxxxx>
>>>> Cc: Omar Sandoval <osandov@xxxxxx>
>>>> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
>>>> ---
>>>> drivers/scsi/scsi_debug.c | 5 +++++
>>>> 1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
>>>> index 24d7496cd9e2..236cfb669df3 100644
>>>> --- a/drivers/scsi/scsi_debug.c
>>>> +++ b/drivers/scsi/scsi_debug.c
>>>> @@ -144,6 +144,7 @@ static const char *sdebug_version_date = "20180128";
>>>> #define DEF_SUBMIT_QUEUES 1
>>>> #define DEF_UUID_CTL 0
>>>> #define JDELAY_OVERRIDDEN -9999
>>>> +#define DEF_USE_BLK_MQ 0
>>>>
>>>> #define SDEBUG_LUN_0_VAL 0
>>>>
>>>> @@ -671,6 +672,7 @@ static bool sdebug_verbose;
>>>> static bool have_dif_prot;
>>>> static bool write_since_sync;
>>>> static bool sdebug_statistics = DEF_STATISTICS;
>>>> +static bool sdebug_use_blk_mq = DEF_USE_BLK_MQ;
>>>>
>>>> static unsigned int sdebug_store_sectors;
>>>> static sector_t sdebug_capacity; /* in sectors */
>>>> @@ -4537,6 +4539,7 @@ module_param_named(vpd_use_hostno, sdebug_vpd_use_hostno, int,
>>>> S_IRUGO | S_IWUSR);
>>>> module_param_named(write_same_length, sdebug_write_same_length, int,
>>>> S_IRUGO | S_IWUSR);
>>>> +module_param_named(use_blk_mq, sdebug_use_blk_mq, bool, S_IRUGO | S_IWUSR);
>>>>
>>>> MODULE_AUTHOR("Eric Youngdale + Douglas Gilbert");
>>>> MODULE_DESCRIPTION("SCSI debug adapter driver");
>>>> @@ -5849,6 +5852,8 @@ static int sdebug_driver_probe(struct device *dev)
>>>> sdebug_driver_template.can_queue = sdebug_max_queue;
>>>> if (sdebug_clustering)
>>>> sdebug_driver_template.use_clustering = ENABLE_CLUSTERING;
>>>> + if (sdebug_use_blk_mq)
>>>> + sdebug_driver_template.force_blk_mq = 1;
>>>> hpnt = scsi_host_alloc(&sdebug_driver_template, sizeof(sdbg_host));
>>>> if (NULL == hpnt) {
>>>> pr_err("scsi_host_alloc failed\n");
>>>>
>>>
>>> Hi,
>>> It is up to others whether this patch goes through. It seems the default
>>> associated with blk_mq may soon change. So two things:
>>> - I'd like to see a MODULE_PARM_DESC(use_blk_mq, ...) entry in patch
>>> - perhaps a 3 valued use_blk_mq: 1 --> force blk_mq; 0 (def) --> accept
>>> default mq setting; -1 --> turn off mq (if possible, if not it's a
>>> NOP)
>>
>> The point is that the parameter is going to be short lived, since the
>> non-mq path will be going away. We don't want drivers exposing this
>> sort of thing, just to remove the option shortly again. Once we're
>> happy with scsi-mq for a release or two, the old code should be
>> deleted. Once that happens, then the option will have no purpose.
>
> Not like other drivers, scsi_debug is a bit special, since it is for
> test purpose. We may switch to scsi_mq at default soon, but the
> whole MQ(block, scsi_mq) code may take ages to be removed,
> maybe never.
It'll definitely get removed, the motivation to do that is strong. I'm
not in violent disagreement with you, my main point is just that it's
going to be a temporary option. Once we flip the switch again and remove
the non-mq path for SCSI a few releases later, then the option is dead.
> We may use the per-driver parameter to test both mq and non-mq
> path, especially for comparing both, then speed up to make MQ code
> mature & stable. That is why I think this patch is very useful.
Also possible with just a reboot, or fiddle with the scsi_mod.use_blk_mq
option...
--
Jens Axboe
