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McKenney" To: Peter Zijlstra Cc: Anna-Maria Gleixner , linux-kernel@vger.kernel.org, Thomas Gleixner , Frederic Weisbecker Subject: Re: sched/core warning triggers on rcu torture test Reply-To: paulmck@linux.vnet.ibm.com References: <20180626163255.GG2458@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180626163255.GG2458@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062617-0072-0000-0000-00000374CBB1 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009259; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01052681; UDB=6.00539676; IPR=6.00830605; MB=3.00021865; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-26 17:46:24 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062617-0073-0000-0000-0000487E88A4 Message-Id: <20180626174826.GB3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-26_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=974 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806260200 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 26, 2018 at 06:32:55PM +0200, Peter Zijlstra wrote: > On Tue, Jun 26, 2018 at 06:16:04PM +0200, Anna-Maria Gleixner wrote: > > Hi, > > > > during rcu torture tests (TREE04 and TREE07) I noticed, that a > > WARN_ON_ONCE() in sched core triggers on a recent 4.18-rc2 based > > kernel (6f0d349d922b ("Merge > > git://git.kernel.org/pub/scm/linux/kernel/git/davem/net")) as well as > > on a 4.17.3. First, I am very glad that I am not the only one running rcutorture! ;-) > > I'm running the tests on a machine with 144 cores: > > > > tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 144 --duration 120 --configs "9*TREE07" > > tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 144 --duration 120 --configs "18*TREE04" > > > > > > The warning was introduced by commit d84b31313ef8 ("sched/isolation: > > Offload residual 1Hz scheduler tick"). > > > > > > Output looks similar for all tests I did (this one is the output of > > the 4.18-rc2 based kernel): > > > > WARNING: CPU: 11 PID: 906 at kernel/sched/core.c:3138 sched_tick_remote+0xb6/0xc0 > > That's nohz_full stuff, is that a normal part of rcutorture? In any > case, is the one housekeeping CPU getting seriously overloaded or > something? Yes, nohz_full is a normal part for rcutorture because RCU has to deal differently with userspace execution in the nohz_full case. I do see this splat (at least when I don't comment it out), but I do share my system with others, so I could easily be overloading the housekeeping vCPUs due to hypervisor preemption. I was intending to dig into this one once I got done consolidating RCU-bh, RCU-preempt, and RCU-sched at Linus's behest. On overloading the housekeeping CPU without outside load, let's look at TREE04 and TREE07 separately. TREE04 uses eight CPUs, and seven of them ("nohz_full=1-7") are nohz_full CPUs, and rcutorture doesn't generate all that large of a callback load. It looks like all 144 CPUs are used in this case (18*8), though RCU enforces idle periods in order to test idle/non-idle transitions. But was there anything else running on the machine at the time? TREE07 uses 16 CPUs, and eight of them ("nohz_full=2-9") are nohz_full CPUs. Again, it looks like all 144 CPUs are used (9*8). I sometimes see this on TASKS03 as well, which uses two CPUs, and one of them ("nohz_full=1") is a nohz_full CPU. If your system is otherwise idle, would it make sense to trace context switches on CPU 0 to see what it is up to? And to do an ftrace_dump() and turn tracing off when the warning triggers as well? Thanx, Paul