[patch 0/4 v2] blk-mq: use percpu_ida to manage tags

[Shaohua Li]

Hi,
blk-mq and percpu_ida use similar algorithm to manage tags. The difference
is when a cpu can't allocate tags, blk-mq will use ipi to purge remote cpu
cache, while percpu-ida directly purges remote cpu cache. In practice, the
percpu-ida approach is much faster when we can't allocate enough for percpu
cache.

This patch makes blk-mq use percpu_ida to manage tags, this avoids some
duplicate code and has better performance as I mentioned. To test the patches,
rebase blk-mq tree to latest kernel first.

The v2 patch updated some log descriptions, fixed one issue Kent pointed out
and added performance data.

Thanks,
Shaohua
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