Messages in this thread | | | Date | Tue, 11 Dec 2018 17:40:16 -0800 | From | Joel Fernandes <> | Subject | Re: [PATCH] srcu: Remove srcu_queue_delayed_work_on() |
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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?
thanks,
- Joel
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