[dm-devel] [PATCH 3/3] block: use a driver-specific handler for the "inflight" value

[Mike Snitzer]

On Fri, Nov 16 2018 at  4:11am -0500,
Christoph Hellwig <hch at infradead.org> wrote:

> On Fri, Nov 16, 2018 at 01:04:19AM +0100, Mikulas Patocka wrote:
> > Device mapper was converted to percpu inflight counters. In order to
> > display the correct values in the "inflight" sysfs file and in
> > /proc/diskstats, we need a custom callback that sums the percpu counters.
> > 
> > The function part_round_stats calculates the number of in-flight I/Os
> > every jiffy and uses this to calculate the counters time_in_queue and
> > io_ticks. In order to avoid excessive memory traffic on systems with high
> > number of CPUs, this functionality is disabled when percpu inflight values
> > are used and the values time_in_queue and io_ticks are calculated
> > differently - the result is less precise.
> 
> And none of that is device mapper specific.  Please submit this code
> to the block layer for use by all make_request based drivers.  Depending
> on what Jens as the maintainers thinkgs of the tradeoffs we can discuss
> if the summing should be on or off by default, or if we maybe even
> need the current code as a fallback.

I agree.

Mikulas, we discussed that the changes would be made to block core.  I
know that makes you less comfortable (I assume because you need to
consider more than DM) but it is the right way forward.

Now that the legacy IO path is gone we have less to consider; these
counters are only impacting bio-based.

Mike