On 11/14/18 5:35 PM, Ming Lei wrote:

On Wed, Nov 14, 2018 at 05:29:54PM +0800, jianchao.wang wrote:

quoted

Hi Ming

On 11/14/18 5:20 PM, Ming Lei wrote:

quoted

On Wed, Nov 14, 2018 at 04:45:29PM +0800, Jianchao Wang wrote:

quoted

When try to issue request directly, if the queue is stopped or
quiesced, 'bypass' will be ignored and return BLK_STS_OK to caller
to avoid it dispatch request again. Then the request will be
inserted with blk_mq_sched_insert_request. This is not correct
for dm-rq case where we should avoid to pass through the underlying
path's io scheduler.

To fix it, use blk_mq_request_bypass_insert to insert the request
to hctx->dispatch when we cannot pass through io scheduler but have
to insert.

Not sure if the current behaviour is wrong, or worth of a fix.

Bypassing io scheduler for dm-rq is only for sake of performance
because there has been io scheduler for dm device already, and we
just don't want to schedule these requests twice.

As comment of commit 157f377beb710e84bd8bc7a3c4475c0674ebebd7
(block: directly insert blk-mq request from blk_insert_cloned_request())

    All said, a request-based DM multipath device's IO scheduler should be
    the only one used -- when the original requests are issued to the
    underlying paths as cloned requests they are inserted directly in the
    underlying dispatch queue(s) rather than through an additional elevator.
    
    But commit bd166ef18 ("blk-mq-sched: add framework for MQ capable IO
    schedulers") switched blk_insert_cloned_request() from using
    blk_mq_insert_request() to blk_mq_sched_insert_request().  Which
    incorrectly added elevator machinery into a call chain that isn't
    supposed to have any.

It sounds like a wrong action.

As I mentioned, it is only for the sake of performance, and IO scheduler
has to be supported on these devices too, for example, one partition may
be under dm-rq, and another partition can be accessed directly.

However, you are fixing the handling when queue is quiesced or stopped.
Under this situation, it is fine to put requests into scheduler queue,
given no performance need to be worried.

OK, I drop this one.

Thanks
Jianchao