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McKenney" To: bigeasy@linutronix.de Cc: linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: EXP rcu: Revert expedited GP parallelization cleverness Reply-To: paulmck@linux.ibm.com MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18110123-0060-0000-0000-000002CB7712 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009968; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000268; SDB=6.01111307; UDB=6.00575901; IPR=6.00891399; MB=3.00023997; MTD=3.00000008; XFM=3.00000015; UTC=2018-11-01 23:30:34 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18110123-0061-0000-0000-0000470E5CFF Message-Id: <20181101233031.GA13002@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-11-01_17:,, 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-1811010196 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > (Commit 258ba8e089db23f760139266c232f01bad73f85c from linux-rcu) > > This commit reverts a series of commits starting with fcc635436501 ("rcu: > Make expedited GPs handle CPU 0 being offline") and its successors, thus > queueing each rcu_node structure's expedited grace-period initialization > work on the first CPU of that rcu_node structure. > > Suggested-by: Sebastian Andrzej Siewior > Signed-off-by: Paul E. McKenney > Signed-off-by: Sebastian Andrzej Siewior > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index 0b2c2ad69629..a0486414edb4 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -472,7 +472,6 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp) > static void sync_rcu_exp_select_cpus(struct rcu_state *rsp, > smp_call_func_t func) > { > - int cpu; > struct rcu_node *rnp; > > trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset")); > @@ -494,13 +493,7 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp, > continue; > } > INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus); > - preempt_disable(); > - cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask); > - /* If all offline, queue the work on an unbound CPU. */ > - if (unlikely(cpu > rnp->grphi)) > - cpu = WORK_CPU_UNBOUND; > - queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work); > - preempt_enable(); > + queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work); > rnp->exp_need_flush = true; > } How about instead changing the earlier "if" statement to read as follows? if (!READ_ONCE(rcu_par_gp_wq) || rcu_scheduler_active != RCU_SCHEDULER_RUNNING || rcu_is_last_leaf_node(rnp) || IS_ENABLED(CONFIG_PREEMPT_RT_FULL)) { /* No workqueues yet or last leaf, do direct call. */ sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work); continue; } This just adds the "|| IS_ENABLED(CONFIG_PREEMPT_RT_FULL)" to the "if" condition. The advantage of this approach is that it leaves the parallelization alone for mainline, and avoids the overhead of the workqueues for -rt. Thanx, Paul