On Thu, Sep 29, 2022 at 07:14:23PM +0300, Sagi Grimberg wrote:

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3. Do you have some performance numbers (we're touching the fast path
here) ?

This is pretty light-weight, accounting is per-cpu and only wrapped by
preemption disable. This is a very small price to pay for what we gain.

It does add up, though, and some environments disable stats to skip the
overhead. At a minimum, you need to add a check for blk_queue_io_stat() before
assuming you need to account for stats.

Instead of duplicating the accounting, could you just have the stats file report
the sum of its hidden devices?

Interesting...

How do you suggest we do that? .collect_stats() callout in fops?

Maybe, yeah. I think we'd need something to enumerate the HIDDEN disks that
make up the multipath device. Only the low-level driver can do that right now,
so perhaps either call into the driver to get all the block_device parts, or
the gendisk needs to maintain a list of those parts itself.