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McKenney" To: Mark Rutland Subject: Re: ftrace hangs waiting for rcu Message-ID: <20220128161547.GL4285@paulmck-ThinkPad-P17-Gen-1> References: <20220127114249.03b1b52b@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: paulmck@kernel.org Cc: linux-s390@vger.kernel.org, Kees Cook , hca@linux.ibm.com, LKML , Steven Rostedt , Ard Biesheuvel , Sven Schnelle , Sachin Sant , Russell King , Andrew Morton , Yinan Liu , linuxppc-dev@lists.ozlabs.org, Ingo Molnar , linux-arm-kernel@lists.infradead.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Fri, Jan 28, 2022 at 04:11:57PM +0000, Mark Rutland wrote: > On Fri, Jan 28, 2022 at 05:08:48PM +0100, Sven Schnelle wrote: > > Hi Mark, > > > > Mark Rutland writes: > > > > > On arm64 I bisected this down to: > > > > > > 7a30871b6a27de1a ("rcu-tasks: Introduce ->percpu_enqueue_shift for dynamic queue selection") > > > > > > Which was going wrong because ilog2() rounds down, and so the shift was wrong > > > for any nr_cpus that was not a power-of-two. Paul had already fixed that in > > > rcu-next, and just sent a pull request to Linus: > > > > > > https://lore.kernel.org/lkml/20220128143251.GA2398275@paulmck-ThinkPad-P17-Gen-1/ > > > > > > With that applied, I no longer see these hangs. > > > > > > Does your s390 test machine have a non-power-of-two nr_cpus, and does that fix > > > the issue for you? > > > > We noticed the PR from Paul and are currently testing the fix. So far > > it's looking good. The configuration where we have seen the hang is a > > bit unusual: > > > > - 16 physical CPUs on the kvm host > > - 248 logical CPUs inside kvm > > Aha! 248 is notably *NOT* a power of two, and in this case the shift would be > wrong (ilog2() would give 7, when we need a shift of 8). > > So I suspect you're hitting the same issue as I was. And apparently no one runs -next on systems having a non-power-of-two number of CPUs. ;-) Thanx, Paul > Thanks, > Mark. > > > - debug kernel both on the host and kvm guest > > > > So things are likely a bit slow in the kvm guest. Interesting is that > > the number of CPUs is even. But maybe RCU sees an odd number of CPUs > > and gets confused before all cpus are brought up. Have to read code/test > > to see whether that could be possible. > > > > Thanks for investigating! > > Sven