Re: [PATCH 3/3] scsi: core: Improve IOPS in case of host-wide tags

[Bart Van Assche <bvanassche@acm.org>]

On 9/11/25 1:15 AM, John Garry wrote:
> this can race with a call to scsi_change_queue_depth() (which may free 
> sdev->budget_map.map), right?
> 
> scsi_change_queue_depth() does not seem to do any queue freezing.

Hi John,

Are there any SCSI devices left about which we care and for which queue
depth tracking is important?

scsi_change_queue_depth() can be called from interrupt context as
follows:

LLD completion interrupt
   scsi_done()
     scsi_done_internal()
       blk_mq_complete_request()
         scsi_complete()
           scsi_decide_disposition()
             scsi_handle_queue_ramp_up()
               scsi_change_queue_depth()

Freezing a request queue requires thread context. Hence, the queue ramp
up queue depth increase would have to happen asynchronously, e.g. via
queue_work().

Here is another call chain:

scsi_error_handler()
   scsi_unjam_host()
     scsi_eh_ready_devs()
       scsi_eh_host_reset()
         scsi_eh_test_devices()
           scsi_eh_tur()
             scsi_send_eh_cmnd()
               scsi_eh_completed_normally()
                 scsi_handle_queue_full()
                   scsi_track_queue_full()
                     scsi_change_queue_depth()

Freezing the request queue probably would result in a deadlock for this 
call chain. So also for this call chain, changing the queue depth would
have to happen asynchronously.

Does anyone see any other options than either making 
scsi_change_queue_depth() a no-op or making all queue depth changes
asynchronously, except the call from scsi_alloc_sdev()?

Thanks,

Bart.