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McKenney" To: Boqun Feng Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 2/2] rcu: Make expedited GPs handle CPU 0 being offline Reply-To: paulmck@linux.vnet.ibm.com References: <20180625224308.GA10064@linux.vnet.ibm.com> <20180625224332.10596-2-paulmck@linux.vnet.ibm.com> <20180626093820.GG2494@hirez.programming.kicks-ass.net> <20180626104447.GG9125@tardis> <20180626114652.GH9125@tardis> <20180626192746.GJ3593@linux.vnet.ibm.com> <20180627024201.GA27704@tardis> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180627024201.GA27704@tardis> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18062716-2213-0000-0000-000002C0C98A X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009264; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01053130; UDB=6.00539945; IPR=6.00831054; MB=3.00021883; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-27 16:13:29 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062716-2214-0000-0000-00005AA0A9A3 Message-Id: <20180627161531.GB3593@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-27_03:,, 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=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806270173 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 27, 2018 at 10:42:01AM +0800, Boqun Feng wrote: > On Tue, Jun 26, 2018 at 12:27:47PM -0700, Paul E. McKenney wrote: > > On Tue, Jun 26, 2018 at 07:46:52PM +0800, Boqun Feng wrote: > > > On Tue, Jun 26, 2018 at 06:44:47PM +0800, Boqun Feng wrote: > > > > On Tue, Jun 26, 2018 at 11:38:20AM +0200, Peter Zijlstra wrote: > > > > > On Mon, Jun 25, 2018 at 03:43:32PM -0700, Paul E. McKenney wrote: > > > > > > + preempt_disable(); > > > > > > + for_each_leaf_node_possible_cpu(rnp, cpu) { > > > > > > + if (cpu_is_offline(cpu)) /* Preemption disabled. */ > > > > > > + continue; > > > > > > > > > > Create for_each_node_online_cpu() instead? Seems a bit pointless to > > > > > iterate possible mask only to then check it against the online mask. > > > > > Just iterate the online mask directly. > > > > > > > > > > Or better yet, write this as: > > > > > > > > > > preempt_disable(); > > > > > cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask); > > > > > if (cpu > rnp->grphi) > > > > > cpu = WORK_CPU_UNBOUND; > > > > > queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work); > > > > > preempt_enable(); > > > > > > > > > > Which is what it appears to be doing. > > > > > > > > > > > > > Make sense! Thanks ;-) > > > > > > > > Applied this and running a TREE03 rcutorture. If all go well, I will > > > > send the updated patch. > > > > > > > > > > So the patch has passed one 30 min run for TREE03 rcutorture. Paul, > > > if it looks good, could you take it for your next spin or pull request > > > in the future? Thanks. > > > > I ended up with the following, mostly just rewording the comment and > > adding a one-liner on the change. Does this work for you? > > Looks good to me. Only one thing I think we need to modify a little, > please see below: > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > commit ef31fa78032536d594630d7bd315d3faf60d98ca > > Author: Boqun Feng > > Date: Fri Jun 15 12:06:31 2018 -0700 > > > > rcu: Make expedited GPs handle CPU 0 being offline > > > > Currently, the parallelized initialization of expedited grace periods uses > > the workqueue associated with each rcu_node structure's ->grplo field. > > This works fine unless that CPU is offline. This commit therefore > > uses the CPU corresponding to the lowest-numbered online CPU, or just > > reports the quiescent states if there are no online CPUs on this rcu_node > > structure. > > better write "or just queue the work on WORK_CPU_UNBOUND if there are > no online CPUs on this rcu_node structure"? Because we currently don't > report the QS directly if all CPU are offline. > > Thoughts? Any objections? If I don't hear any by tomorrow morning (Pacific Time), I will make this change. Thanx, Paul > Regards, > Boqun > > > > > Note that this patch uses cpu_is_offline() instead of the usual > > approach of checking bits in the rcu_node structure's ->qsmaskinitnext > > field. This is safe because preemption is disabled across both the > > cpu_is_offline() check and the call to queue_work_on(). > > > > Signed-off-by: Boqun Feng > > [ paulmck: Disable preemption to close offline race window. ] > > Signed-off-by: Paul E. McKenney > > [ paulmck: Apply Peter Zijlstra feedback on CPU selection. ] > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index c6385ee1af65..b3df3b770afb 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -472,6 +472,7 @@ 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")); > > @@ -493,7 +494,13 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp, > > continue; > > } > > INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus); > > - queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work); > > + 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(); > > rnp->exp_need_flush = true; > > } > > > >