On 25/06/2020 06:24, kernelci.org bot wrote: > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * > * This automated bisection report was sent to you on the basis * > * that you may be involved with the breaking commit it has * > * found. No manual investigation has been done to verify it, * > * and the root cause of the problem may be somewhere else. * > * * > * If you do send a fix, please include this trailer: * > * Reported-by: "kernelci.org bot" <bot@xxxxxxxxxxxx> * > * * > * Hope this helps! * > * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * > > stable-rc/linux-5.6.y bisection: baseline.dmesg.crit on bcm2837-rpi-3-b > > Summary: > Start: 61aba373f570 Linux 5.6.19 > Plain log: https://storage.kernelci.org/stable-rc/linux-5.6.y/v5.6.19/arm64/defconfig/gcc-8/lab-baylibre/baseline-bcm2837-rpi-3-b.txt > HTML log: https://storage.kernelci.org/stable-rc/linux-5.6.y/v5.6.19/arm64/defconfig/gcc-8/lab-baylibre/baseline-bcm2837-rpi-3-b.html > Result: 9cf5d5444c78 Revert "cgroup: Add memory barriers to plug cgroup_rstat_updated() race window" > > Checks: > revert: PASS > verify: PASS > > Parameters: > Tree: stable-rc > URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable-rc.git > Branch: linux-5.6.y > Target: bcm2837-rpi-3-b > CPU arch: arm64 > Lab: lab-baylibre > Compiler: gcc-8 > Config: defconfig > Test case: baseline.dmesg.crit The "crit" kernel message is: [ 17.536674] hwmon hwmon1: Undervoltage detected! which is a known intermittent issue on rpi-3-b that is most likely to be unrelated to the patch found by this bisection. Maybe the bisection landed on this patch because it makes the undervoltage more likely by a side effect, if it affects timing or the binary size or addresses in very subtle ways. But even then it wouldn't be a regression. We should stop running bisections on rpi-3-b, at least for this kernel error message, until it has been fixed to avoid future false positives. Sorry for the noise. Guillaume > Breaking commit found: > > ------------------------------------------------------------------------------- > commit 9cf5d5444c78c14bb9f90dd21cef361ee14ba64b > Author: Tejun Heo <tj@xxxxxxxxxx> > Date: Thu Apr 9 14:55:35 2020 -0400 > > Revert "cgroup: Add memory barriers to plug cgroup_rstat_updated() race window" > > [ Upstream commit d8ef4b38cb69d907f9b0e889c44d05fc0f890977 ] > > This reverts commit 9a9e97b2f1f2 ("cgroup: Add memory barriers to plug > cgroup_rstat_updated() race window"). > > The commit was added in anticipation of memcg rstat conversion which needed > synchronous accounting for the event counters (e.g. oom kill count). However, > the conversion didn't get merged due to percpu memory overhead concern which > couldn't be addressed at the time. > > Unfortunately, the patch's addition of smp_mb() to cgroup_rstat_updated() > meant that every scheduling event now had to go through an additional full > barrier and Mel Gorman noticed it as 1% regression in netperf UDP_STREAM test. > > There's no need to have this barrier in tree now and even if we need > synchronous accounting in the future, the right thing to do is separating that > out to a separate function so that hot paths which don't care about > synchronous behavior don't have to pay the overhead of the full barrier. Let's > revert. > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Reported-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Link: http://lkml.kernel.org/r/20200409154413.GK3818@xxxxxxxxxxxxxxxxxxx > Cc: v4.18+ > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> > > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c > index 6f87352f8219..41ca996568df 100644 > --- a/kernel/cgroup/rstat.c > +++ b/kernel/cgroup/rstat.c > @@ -33,12 +33,9 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu) > return; > > /* > - * Paired with the one in cgroup_rstat_cpu_pop_updated(). Either we > - * see NULL updated_next or they see our updated stat. > - */ > - smp_mb(); > - > - /* > + * Speculative already-on-list test. This may race leading to > + * temporary inaccuracies, which is fine. > + * > * Because @parent's updated_children is terminated with @parent > * instead of NULL, we can tell whether @cgrp is on the list by > * testing the next pointer for NULL. > @@ -134,13 +131,6 @@ static struct cgroup *cgroup_rstat_cpu_pop_updated(struct cgroup *pos, > *nextp = rstatc->updated_next; > rstatc->updated_next = NULL; > > - /* > - * Paired with the one in cgroup_rstat_cpu_updated(). > - * Either they see NULL updated_next or we see their > - * updated stat. > - */ > - smp_mb(); > - > return pos; > } > ------------------------------------------------------------------------------- > > > Git bisection log: > > ------------------------------------------------------------------------------- > git bisect start > # good: [7111951b8d4973bda27ff663f2cf18b663d15b48] Linux 5.6 > git bisect good 7111951b8d4973bda27ff663f2cf18b663d15b48 > # bad: [61aba373f5708f2aebc3f72078e51949a068aa6f] Linux 5.6.19 > git bisect bad 61aba373f5708f2aebc3f72078e51949a068aa6f > # good: [0bc4d081e87ef8fd43baeed7141267e558896f86] RDMA/cm: Fix an error check in cm_alloc_id_priv() > git bisect good 0bc4d081e87ef8fd43baeed7141267e558896f86 > # good: [0deceb85de592b89f6476e2bfcb5321a95d23126] dmaengine: owl: Use correct lock in owl_dma_get_pchan() > git bisect good 0deceb85de592b89f6476e2bfcb5321a95d23126 > # bad: [566f16e2d62fb3817529a05147ef46046ecffe6a] io_uring: initialize ctx->sqo_wait earlier > git bisect bad 566f16e2d62fb3817529a05147ef46046ecffe6a > # good: [23952755bc2f839844b5a60c29620cce98c9b09d] riscv: stacktrace: Fix undefined reference to `walk_stackframe' > git bisect good 23952755bc2f839844b5a60c29620cce98c9b09d > # good: [be31933db49c8e3d81b96ee6bc9933d2adafe13d] drm/amd/display: Fix potential integer wraparound resulting in a hang > git bisect good be31933db49c8e3d81b96ee6bc9933d2adafe13d > # good: [ee899046f79db390650e546446a7a0196788d39d] esp6: get the right proto for transport mode in esp6_gso_encap > git bisect good ee899046f79db390650e546446a7a0196788d39d > # good: [bdd07c33a3be1896ec1982fab1a2baf986fbe4dc] netfilter: conntrack: comparison of unsigned in cthelper confirmation > git bisect good bdd07c33a3be1896ec1982fab1a2baf986fbe4dc > # bad: [46f4a75fb5034c8d80a5388fe3bb46a8ed05df5b] ARC: [plat-eznps]: Restrict to CONFIG_ISA_ARCOMPACT > git bisect bad 46f4a75fb5034c8d80a5388fe3bb46a8ed05df5b > # good: [960a4cc3ec49f8292d0f837f0a6b28b03c54f042] Linux 5.6.16 > git bisect good 960a4cc3ec49f8292d0f837f0a6b28b03c54f042 > # bad: [9cf5d5444c78c14bb9f90dd21cef361ee14ba64b] Revert "cgroup: Add memory barriers to plug cgroup_rstat_updated() race window" > git bisect bad 9cf5d5444c78c14bb9f90dd21cef361ee14ba64b > # good: [2df9b670ad1a2120f54982e1b1dfd17a6ce3cc0c] x86/syscalls: Revert "x86/syscalls: Make __X32_SYSCALL_BIT be unsigned long" > git bisect good 2df9b670ad1a2120f54982e1b1dfd17a6ce3cc0c > # first bad commit: [9cf5d5444c78c14bb9f90dd21cef361ee14ba64b] Revert "cgroup: Add memory barriers to plug cgroup_rstat_updated() race window" > ------------------------------------------------------------------------------- >
