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McKenney" To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , Steven Rostedt Subject: Re: [RFC] rcu: Avoid unnecessary softirq when system is idle Reply-To: paulmck@linux.ibm.com References: <20190119161418.231042-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190119161418.231042-1-joel@joelfernandes.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19012001-0040-0000-0000-000004B48D57 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010439; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000275; SDB=6.01148964; UDB=6.00593882; IPR=6.00929272; MB=3.00025206; MTD=3.00000008; XFM=3.00000015; UTC=2019-01-20 01:07:36 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19012001-0041-0000-0000-000008BF998B Message-Id: <20190120010732.GH4240@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-01-19_15:,, 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=864 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1901200007 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 19, 2019 at 11:14:18AM -0500, Joel Fernandes (Google) wrote: > When there are no callbacks pending on an idle system, I noticed that > RCU softirq is continuously firing. During this the cpu_no_qs is set to > false, and core_needs_qs is set to true indefinitely. This causes > rcu_process_callbacks to be repeatedly called, even though the node > corresponding to the CPU has that CPU's mask bit cleared and the system > is idle. I believe the race is when such mask clearing is done during > idle CPU scan of the quiescent state forcing stage in the kthread > instead of the softirq. Since the rnp mask is cleared, but the flags on > the CPU's rdp are not cleared, the CPU thinks it still needs to report > to core RCU. > > Cure this by clearing the core_needs_qs flag when the CPU detects that > its node is already updated which will avoid the unwanted softirq raises > to the benefit of real-time systems. > > Test: Ran rcutorture for various tree RCU configs. > > Signed-off-by: Joel Fernandes (Google) Good catch, queued, thank you! Thanx, Paul > --- > kernel/rcu/tree.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 9180158756d2..96ad80c76b15 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2273,6 +2273,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_data *rdp) > } > mask = rdp->grpmask; > if ((rnp->qsmask & mask) == 0) { > + rdp->core_needs_qs = false; > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > } else { > rdp->core_needs_qs = false; > -- > 2.20.1.321.g9e740568ce-goog >