From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: Re: WARNING: at /home/konrad/linux-linus/kernel/time/tick-sched.c:935 tick_nohz_idle_exit+0x195/0x1b0() on v3.10-rc3 Date: Thu, 30 May 2013 22:05:46 +0200 (CEST) Message-ID: References: <20130530182948.GA11612@phenom.dumpdata.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Return-path: In-Reply-To: <20130530182948.GA11612@phenom.dumpdata.com> Sender: linux-kernel-owner@vger.kernel.org To: Konrad Rzeszutek Wilk Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org On Thu, 30 May 2013, Konrad Rzeszutek Wilk wrote: > [ 40.085841] WARNING: at /home/konrad/linux-linus/kernel/time/tick-sched.c:935 tick_nohz_idle_exit+0x195/0x1b0() > > which I presume is b/c the code does not expect to be run _after_ it has > offlined. However, under the PV code, the mechanism is that that a CPU > that has been offlined, can resume (if it is onlined). If you look at: > > 445 static void __cpuinit xen_play_dead(void) /* used only with HOTPLUG_CPU */ > 446 { > 447 play_dead_common(); > 448 HYPERVISOR_vcpu_op(VCPUOP_down, smp_processor_id(), NULL); > 449 cpu_bringup(); > 450 } > > That is called right after the CPU is put to sleep and the hypercall > VCPUOP_down blocks - until the CPU is brough back up. And which point > we end up calling cpu_bringup - which sets up the clockevets, timers, etc. > > I am wondering if part of this is that the ts->inidle gets reset > b/c we end up resetting all the timers but then when xen_play_dead > exits, it ends up right back in the cpu_idle_loop() loop - and we > call tick_nohz_idle_exit(). > > Thoughts? cpu_dead() is definitely not expected to return after the cpu has been declared dead. I should have put a big fat warning into the generic idle loop for this :) The reason why you get that warning only now is commit 4b0c0f294 (tick: Cleanup NOHZ per cpu data on cpu down), which is btw. targeted for stable as well. We can't revert the above commit as it fixes a long standing nastiness, so for now until I come around to make the idle loop return on cpu down you probably need to call tick_nohz_idle_enter() before returning from play_dead(). Thanks, tglx