On Wed, Mar 10, 2021 at 10:43:33AM -0800, Linus Torvalds wrote: > Just a note to the stable tree: this commit has been reverted > upstream, because it causes a huge performance drop (admittedly on a > load and setup that may not be all that relevant to most people). > > It was applied to 4.4, 4.9 and 4.12, because the commit it was marked > as "fixing" is from 2012, but it turns out that the early exit from > the loop in that commit was very much intentional, and very much shows > up on scalability benchmarks. > > I don't think this is likely to be a big deal for the stable kernels - > we're basically talking tuning for special cases, and while it is > reverted in my tree now, the "correct" thing to do is likely to be a > bit more flexible than either "exit loop immediately" or "loop for as > long as we have contention". > > In practice, most machines probably won't see either case - or it will > at least be rare enough that you can't tell. > > The machine that reports a huge performance drop was a multi-socket > machine under fairly extreme conditions, and these contention issues > are often close to exponential - a smaller machine (or a slighly less > extreme load) would never see the issue at all either way. > > See > > https://lore.kernel.org/lkml/20210301080404.GF12822@xsang-OptiPlex-9020/ > > for details if you care. I don't think this has to necessarily be > undone in the stable trees, this email is more of an incidental note > just as a heads-up. Thanks for the details, I'll look into reverting it in a future stable release. greg k-h
