Messages in this thread | | | Date | Fri, 7 Oct 2005 09:23:33 +0200 | From | Jens Axboe <> | Subject | Re: [RFC] add sysfs to dynamically control blk request tag maintenance |
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On Thu, Oct 06 2005, Chen, Kenneth W wrote: > blk_queue_start_tag and blk_queue_end_tag are called for tagging > I/O to scsi device that is capable of tcq. blk_queue_find_tag is > a function that utilizes the tag information built up on every I/O. > > However, there aren't many consumers for blk_queue_find_tag, except > NCR53c700 and tekram-dc390. Vast majority of scsi drivers don't > use these tag currently. So why bother build them at the beginning > of an I/O and then tear it all down at the end, all doing hard work > but no other functions in the kernel appears to care. > > Is there another big scheme in the works to use these tags? If not, > I'd like to propose we add a sysfs attribute to dynamically control > whether kernel maintains blk request tag or not. This has performance > advantage that we don't needlessly waste CPU cycle on things we throw > away without using them. Would the following patch be acceptable? > Comments?
I don't understand the need for this patch - the generic tagging is only used if the SCSI LLD indicated it wanted it by issuing a scsi_activate_tcq(). So blk_queue_start_tag() is only called if the LLD already did a scsi_activate_tcq(), and blk_queue_end_tag() is only called if the rq is block layer tagged. blk_queue_find_tag() is only used with direct use of scsi_find_tag(), a function that should (and is) only usable by users of the generic tagging already.
So please, a description of what problem you are trying to solve would be appreciated :-)
-- Jens Axboe
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