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McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 01/14] rcu: Stop expedited grace periods from relying on stop-machine Date: Sun, 11 Nov 2018 11:28:51 -0800 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20181111192839.GA32144@linux.ibm.com> References: <20181111192839.GA32144@linux.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18111119-2213-0000-0000-00000315A744 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010028; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01116014; UDB=6.00578729; IPR=6.00896114; MB=3.00024114; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-11 19:29:09 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18111119-2214-0000-0000-00005C3866AC Message-Id: <20181111192904.3199-1-paulmck@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-11_13:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1811110185 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" The CPU-selection code in sync_rcu_exp_select_cpus() disables preemption to prevent the cpu_online_mask from changing. However, this relies on the stop-machine mechanism in the CPU-hotplug offline code, which is not desirable (it would be good to someday remove the stop-machine mechanism). This commit therefore instead uses the relevant leaf rcu_node structure's ->ffmask, which has a bit set for all CPUs that are fully functional. A given CPU's bit is cleared very early during offline processing by rcutree_offline_cpu() and set very late during online processing by rcutree_online_cpu(). Therefore, if a CPU's bit is set in this mask, and preemption is disabled, we have to be before the synchronize_sched() in the CPU-hotplug offline code, which means that the CPU is guaranteed to be workqueue-ready throughout the duration of the enclosing preempt_disable() region of code. This also has the side-effect of using WORK_CPU_UNBOUND if all the CPUs for this leaf rcu_node structure are offline, which is an acceptable difference in behavior. Reported-by: Sebastian Andrzej Siewior Signed-off-by: Paul E. McKenney --- kernel/rcu/tree_exp.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h index 8d18c1014e2b..e669ccf3751b 100644 --- a/kernel/rcu/tree_exp.h +++ b/kernel/rcu/tree_exp.h @@ -450,10 +450,12 @@ static void sync_rcu_exp_select_cpus(smp_call_func_t func) } INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus); preempt_disable(); - cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask); + cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1); /* If all offline, queue the work on an unbound CPU. */ - if (unlikely(cpu > rnp->grphi)) + if (unlikely(cpu > rnp->grphi - rnp->grplo)) cpu = WORK_CPU_UNBOUND; + else + cpu += rnp->grplo; queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work); preempt_enable(); rnp->exp_need_flush = true; -- 2.17.1