Messages in this thread | | | Date | Wed, 12 Dec 2018 07:55:46 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] srcu: Remove srcu_queue_delayed_work_on() |
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On Wed, Dec 12, 2018 at 10:05:19PM +0800, Boqun Feng wrote: > Hi Joel, > > On Tue, Dec 11, 2018 at 05:40:16PM -0800, Joel Fernandes wrote: > > On Tue, Dec 11, 2018 at 12:12:38PM +0100, Sebastian Andrzej Siewior wrote: > > > srcu_queue_delayed_work_on() disables preemption (and therefore CPU > > > hotplug in RCU's case) and then checks based on its own accounting if a > > > CPU is online. If the CPU is online it uses queue_delayed_work_on() > > > otherwise it fallbacks to queue_delayed_work(). > > > The problem here is that queue_work() on -RT does not work with disabled > > > preemption. > > > > > > queue_work_on() works also on an offlined CPU. queue_delayed_work_on() > > > has the problem that it is possible to program a timer on an offlined > > > CPU. This timer will fire once the CPU is online again. But until then, > > > the timer remains programmed and nothing will happen. > > > Add a local timer which will fire (as requested per delay) on the local > > > CPU and then enqueue the work on the specific CPU. > > > > > > RCUtorture testing with SRCU-P for 24h showed no problems. > > > > > > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > > > --- > > > include/linux/srcutree.h | 3 ++- > > > kernel/rcu/srcutree.c | 57 ++++++++++++++++++---------------------- > > > kernel/rcu/tree.c | 4 --- > > > kernel/rcu/tree.h | 8 ------ > > > 4 files changed, 27 insertions(+), 45 deletions(-) > > > > > > diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h > > > index 6f292bd3e7db7..0faa978c98807 100644 > > > --- a/include/linux/srcutree.h > > > +++ b/include/linux/srcutree.h > > > @@ -45,7 +45,8 @@ struct srcu_data { > > > unsigned long srcu_gp_seq_needed; /* Furthest future GP needed. */ > > > unsigned long srcu_gp_seq_needed_exp; /* Furthest future exp GP. */ > > > bool srcu_cblist_invoking; /* Invoking these CBs? */ > > > - struct delayed_work work; /* Context for CB invoking. */ > > > + struct timer_list delay_work; /* Delay for CB invoking */ > > > + struct work_struct work; /* Context for CB invoking. */ > > > struct rcu_head srcu_barrier_head; /* For srcu_barrier() use. */ > > > struct srcu_node *mynode; /* Leaf srcu_node. */ > > > unsigned long grpmask; /* Mask for leaf srcu_node */ > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > > index 3600d88d8956b..7f041f2435df9 100644 > > > --- a/kernel/rcu/srcutree.c > > > +++ b/kernel/rcu/srcutree.c > > > @@ -58,6 +58,7 @@ static bool __read_mostly srcu_init_done; > > > static void srcu_invoke_callbacks(struct work_struct *work); > > > static void srcu_reschedule(struct srcu_struct *ssp, unsigned long delay); > > > static void process_srcu(struct work_struct *work); > > > +static void srcu_delay_timer(struct timer_list *t); > > > > > > /* Wrappers for lock acquisition and release, see raw_spin_lock_rcu_node(). */ > > > #define spin_lock_rcu_node(p) \ > > > @@ -156,7 +157,8 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static) > > > snp->grphi = cpu; > > > } > > > sdp->cpu = cpu; > > > - INIT_DELAYED_WORK(&sdp->work, srcu_invoke_callbacks); > > > + INIT_WORK(&sdp->work, srcu_invoke_callbacks); > > > + timer_setup(&sdp->delay_work, srcu_delay_timer, 0); > > > sdp->ssp = ssp; > > > sdp->grpmask = 1 << (cpu - sdp->mynode->grplo); > > > if (is_static) > > > @@ -386,13 +388,19 @@ void _cleanup_srcu_struct(struct srcu_struct *ssp, bool quiesced) > > > } else { > > > flush_delayed_work(&ssp->work); > > > } > > > - for_each_possible_cpu(cpu) > > > + for_each_possible_cpu(cpu) { > > > + struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu); > > > + > > > if (quiesced) { > > > - if (WARN_ON(delayed_work_pending(&per_cpu_ptr(ssp->sda, cpu)->work))) > > > + if (WARN_ON(timer_pending(&sdp->delay_work))) > > > + return; /* Just leak it! */ > > > + if (WARN_ON(work_pending(&sdp->work))) > > > return; /* Just leak it! */ > > > } else { > > > - flush_delayed_work(&per_cpu_ptr(ssp->sda, cpu)->work); > > > + del_timer_sync(&sdp->delay_work); > > > + flush_work(&sdp->work); > > > } > > > + } > > > if (WARN_ON(rcu_seq_state(READ_ONCE(ssp->srcu_gp_seq)) != SRCU_STATE_IDLE) || > > > WARN_ON(srcu_readers_active(ssp))) { > > > pr_info("%s: Active srcu_struct %p state: %d\n", > > > @@ -463,39 +471,23 @@ static void srcu_gp_start(struct srcu_struct *ssp) > > > WARN_ON_ONCE(state != SRCU_STATE_SCAN1); > > > } > > > > > > -/* > > > - * Track online CPUs to guide callback workqueue placement. > > > - */ > > > -DEFINE_PER_CPU(bool, srcu_online); > > > > > > -void srcu_online_cpu(unsigned int cpu) > > > +static void srcu_delay_timer(struct timer_list *t) > > > { > > > - WRITE_ONCE(per_cpu(srcu_online, cpu), true); > > > + struct srcu_data *sdp = container_of(t, struct srcu_data, delay_work); > > > + > > > + queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work); > > > } > > > > > > -void srcu_offline_cpu(unsigned int cpu) > > > -{ > > > - WRITE_ONCE(per_cpu(srcu_online, cpu), false); > > > -} > > > - > > > -/* > > > - * Place the workqueue handler on the specified CPU if online, otherwise > > > - * just run it whereever. This is useful for placing workqueue handlers > > > - * that are to invoke the specified CPU's callbacks. > > > - */ > > > -static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq, > > > - struct delayed_work *dwork, > > > +static void srcu_queue_delayed_work_on(struct srcu_data *sdp, > > > unsigned long delay) > > > { > > > - bool ret; > > > + if (!delay) { > > > + queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work); > > > + return; > > > + } > > > > > > - preempt_disable(); > > > - if (READ_ONCE(per_cpu(srcu_online, cpu))) > > > - ret = queue_delayed_work_on(cpu, wq, dwork, delay); > > > - else > > > - ret = queue_delayed_work(wq, dwork, delay); > > > - preempt_enable(); > > > > The deleted code looks like 'cpu' could be offlined. > > > > Question for my clarification: According to sync_rcu_exp_select_cpus, we have > > to disable preemption before calling queue_work_on to ensure the CPU is online. > > > > Also same is said in Boqun's presentation on the topic: > > https://linuxplumbersconf.org/event/2/contributions/158/attachments/68/79/workqueue_and_cpu_hotplug.pdf > > > > Calling queue_work_on on an offline CPU sounds like could be problematic. So > > in your patch, don't you still need to disable preemption around > > queue_work_on, or the very least check if said CPU is online before calling > > queue_work_on? > > > > I should be the one who answers this ;-) > > So I found something after my LPC topic, that is queue_work_on() may > work well even if racing with a cpu offline. The reason is that in the > cpu offline hook for workqueue only switches the percpu pool into an > unbound one, so as long as the related cpu has been online once, we are > fine here. > > Please note this is only my own analysis, and I'm the one who told > Sebastian and Paul about this. > > I should send an email to check with workqueue maintainers about this, > but I didn't find the time. Sorry about this.. But let's do this right > now, while we at it. > > (Cc TJ) > > So Jiangshan and TJ, what's your opion on this one? If we call a > queue_work_on() at a place where that target cpu may be offlined, I > think we have the guarantee that the work will be eventually executed > even if the cpu is never online again, right? In other words, if a cpu > has been online once, queue_work_on() on it will be free from racing > with cpu hotplug. > > Am I right about this, or did I miss something subtle?
Well, what I was relying on was Thomas Gleixner's assertion that it should work and would be fixed if it didn't. ;-)
Thanx, Paul
> Regards, > Boqun > > > thanks, > > > > - Joel > >
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