On Fri, Oct 20, 2023 at 10:27 AM Luiz Capitulino <luizcap@xxxxxxxxxx> wrote: > > Hi, > > As reported before[1], we found another regression in 6.1 when doing > performance comparisons with 5.10. This one is caused by CONFIG_DEBUG_PREEMPT > being enabled by default by the following upstream commit if you have the > right config dependencies enabled (commit is introduced in v5.16-rc1): > > """ > commit c597bfddc9e9e8a63817252b67c3ca0e544ace26 > Author: Frederic Weisbecker <frederic@xxxxxxxxxx> > Date: Tue Sep 14 12:31:34 2021 +0200 > > sched: Provide Kconfig support for default dynamic preempt mode > """ > > We found up to 8% performance improvement with CONFIG_DEBUG_PREEMPT > disabled in different perf benchmarks (including UnixBench process > creation and redis). The root cause is explained in the commit log > below which is merged in 6.3 and applies (almost) clealy on 6.1.59. Oh, I should've sent it to the stable. Thanks for sending it! Yes, DEBUG_PREEMPT was unintentionally enabled after the introduction of PREEMPT_DYNAMIC. It was already enabled by default for PREEMPTION=y kernels but PREEMPT_DYNAMIC always enables PREEMPT_BUILD (and hence PREEMPTION) so distros that were using PREEMPT_VOLUNTARY are silently affected by that. It looks appropriate to be backported to the stable tree (to me). Hmm but I think it should be backported to 5.15 too? > """ > commit cc6003916ed46d7a67d91ee32de0f9138047d55f > Author: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> > Date: Sat Jan 21 12:39:42 2023 +0900 > > lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default > > In workloads where this_cpu operations are frequently performed, > enabling DEBUG_PREEMPT may result in significant increase in > runtime overhead due to frequent invocation of > __this_cpu_preempt_check() function. > > This can be demonstrated through benchmarks such as hackbench where this > configuration results in a 10% reduction in performance, primarily due to > the added overhead within memcg charging path. > """ > > [1] https://lore.kernel.org/stable/010edf5a-453d-4c98-9c07-12e75d3f983c@xxxxxxxxxx/
