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

On Fri, 16 Nov 2018, Jens Axboe wrote:

On 11/16/18 6:55 AM, Mike Snitzer wrote:

quoted

On Fri, Nov 16 2018 at  4:11am -0500,
Christoph Hellwig [off-list ref] wrote:

quoted

On Fri, Nov 16, 2018 at 01:04:19AM +0100, Mikulas Patocka wrote:

quoted

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.

Agree, either the new code is good enough to be used in general, and
then it should be generally used, or it should not exist in the first
place. We've always worked very hard to provide the most efficient
helpers and infrastructure we can in the block layer itself, so that
drivers don't have to reinvent the wheel.

-- 
Jens Axboe

I have generalized the per-cpu changes (so that all bio-based block 
devices use per-cpu in_flight counters) and I've made patches for the 
for-4.21/block branch in your git. I'm sending them.

Mikulas