Re: [PATCH v4 12/13] blk-mq: Use shared tags for shared sbitmap support

[Hannes Reinecke]

On 9/24/21 10:28 AM, John Garry wrote:
> Currently we use separate sbitmap pairs and active_queues atomic_t for
> shared sbitmap support.
>
> However a full sets of static requests are used per HW queue, which is
> quite wasteful, considering that the total number of requests usable at
> any given time across all HW queues is limited by the shared sbitmap depth.
>
> As such, it is considerably more memory efficient in the case of shared
> sbitmap to allocate a set of static rqs per tag set or request queue, and
> not per HW queue.
>
> So replace the sbitmap pairs and active_queues atomic_t with a shared
> tags per tagset and request queue, which will hold a set of shared static
> rqs.
>
> Since there is now no valid HW queue index to be passed to the blk_mq_ops
> .init and .exit_request callbacks, pass an invalid index token. This
> changes the semantics of the APIs, such that the callback would need to
> validate the HW queue index before using it. Currently no user of shared
> sbitmap actually uses the HW queue index (as would be expected).
>
> Continue to use term "shared sbitmap" for now, as the meaning is known.
>
> Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> ---
>   block/blk-mq-sched.c   | 82 ++++++++++++++++++-------------------
>   block/blk-mq-tag.c     | 61 ++++++++++------------------
>   block/blk-mq-tag.h     |  6 +--
>   block/blk-mq.c         | 91 +++++++++++++++++++++++-------------------
>   block/blk-mq.h         |  5 ++-
>   include/linux/blk-mq.h | 15 ++++---
>   include/linux/blkdev.h |  3 +-
>   7 files changed, 125 insertions(+), 138 deletions(-)
The overall idea to keep the full request allocation per queue was to 
ensure memory locality for the requests themselves.
When moving to a shared request structure we obviously loose that feature.

But I'm not sure if that matters here; the performance impact might be 
too small to be measurable, seeing that we'll be most likely bound by 
hardware latencies anyway.

Nevertheless: have you tested for performance regressions with this 
patchset?
I'm especially thinking of Kashyaps high-IOPS megaraid setup; if there 
is a performance impact that'll be likely scenario where we can measure it.

But even if there is a performance impact this patchset might be 
worthwhile, seeing that it'll reduce the memory footprint massively.

Cheers,

Hannes
--
Dr. Hannes Reinecke                Kernel Storage Architect
hare@xxxxxxx                              +49 911 74053 688
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