On Tue, Jul 28, 2020 at 08:54:27AM +0100, John Garry wrote:

On 24/07/2020 03:47, Ming Lei wrote:

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On Thu, Jul 23, 2020 at 06:29:01PM +0100, John Garry wrote:

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As I see, since megaraid will have 1:1 mapping of CPU to hw queue, will
there only ever possibly a single bit set in ctx_map? If so, it seems a
waste to always check every sbitmap map. But adding logic for this may
negate any possible gains.

It really depends on min and max cpu id in the map, then sbitmap
depth can be reduced to (max - min + 1). I'd suggest to double check that
cost of sbitmap_any_bit_set() really matters.

Hi Ming,

I'm not sure that reducing the search range would help much, as we still
need to load some indexes of map[], and at best this may be reduced from 2/3
-> 1 elements, depending on nr_cpus.

I believe you misunderstood my idea, and you have to think it from implementation
viewpoint.

The only workable way is to store the min cpu id as 'offset' and set the sbitmap
depth as (max - min + 1), isn't it? Then the actual cpu id can be figured out via
'offset' + nr_bit. And the whole indexes are just spread on the actual depth. BTW,
max & min is the max / min cpu id in hctx->cpu_map. So we can improve not only on 1:1,
and I guess most of MQ cases can benefit from the change, since it shouldn't be usual
for one ctx_map to cover both 0 & nr_cpu_id - 1.

Meantime, we need to allocate the sbitmap dynamically.

OK, so dynamically allocating the sbitmap could be good. I was thinking
previously that we still allocate for nr_cpus size, and search a limited
range - but this would have heavier runtime overhead.

So if you really think that this may have some value, then let me know, so
we can look to take it forward.

Forget to mention, the in-tree code has been this shape for long
time, please see sbitmap_resize() called from blk_mq_map_swqueue().

Another update is that V4 of 'scsi: core: only re-run queue in scsi_end_request()
if device queue is busy' is quite hard to implement since commit b4fd63f42647110c9
("Revert "scsi: core: run queue if SCSI device queue isn't ready and queue is idle").


Thanks,
Ming