On 2014-09-22 10:25, Bart Van Assche wrote:
On 22/09/2014 8:37, Christoph Hellwig wrote:
One thing that is missing is generation multiqueue-aware tags at the
blk-mq level, which should be as simple as always adding a queue
prefix in the tag allocation code.
Hello Christoph,
Adding a queue prefix in the tag allocation code is an interesting idea.
Encoding the hardware context index in the upper bits of the 'tag' field
in 'struct request' is something I have considered. The reason I have
not done that is because I think several block drivers assume that the
rq->tag field is a number in the range [0..queue_depth-1]. Here is just
one example from the mtip32xx driver:
fis->sect_count = ((rq->tag << 3) | (rq->tag >> 5));
Most drivers assume that the tag is within a certain range, the queue
prefix would only work on drivers where the tag number is just some
arbitrary "cookie". So for SCSI it should work, and I don't think we
need it anywhere else.
Alternatively, we can wrap tag retrieval in some function to mask off
the queue prefix for the cases where we just want an index.
--
Jens Axboe
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