On Fri, 16 Nov 2018, Jens Axboe wrote: > On 11/16/18 6:55 AM, Mike Snitzer wrote: > > On Fri, Nov 16 2018 at 4:11am -0500, > > Christoph Hellwig <hch@xxxxxxxxxxxxx> 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. > > 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
