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Message-ID: <20230223171324.GB2948950@paulmck-ThinkPad-P17-Gen-1>
Date: Thu, 23 Feb 2023 09:13:24 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: "Zhang, Qiang1" <qiang1.zhang@...el.com>,
"Zhuo, Qiuxu" <qiuxu.zhuo@...el.com>, frederic@...nel.org,
quic_neeraju@...cinc.com, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu-tasks: Directly invoke rcuwait_wake_up() in
call_rcu_tasks_generic()
On Thu, Feb 23, 2023 at 11:57:54AM -0500, Joel Fernandes wrote:
>
>
> > On Feb 23, 2023, at 11:10 AM, Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > On Thu, Feb 23, 2023 at 08:43:05AM +0000, Zhang, Qiang1 wrote:
> >>> From: Zqiang <qiang1.zhang@...el.com>
> >>> Sent: Thursday, February 23, 2023 2:30 PM
> >>> To: paulmck@...nel.org; frederic@...nel.org; quic_neeraju@...cinc.com;
> >>> joel@...lfernandes.org
> >>> Cc: rcu@...r.kernel.org; linux-kernel@...r.kernel.org
> >>> Subject: [PATCH] rcu-tasks: Directly invoke rcuwait_wake_up() in
> >>> call_rcu_tasks_generic()
> >>>
> >>> According to commit '3063b33a347c ("Avoid raw-spinlocked wakeups from
> >>> call_rcu_tasks_generic()")', the grace-period kthread is delayed to wakeup
> >>> using irq_work_queue() is because if the caller of
> >>> call_rcu_tasks_generic() holds a raw spinlock, when the kernel is built with
> >>> CONFIG_PROVE_RAW_LOCK_NESTING=y, due to a spinlock will be hold in
> >>> wake_up(), so the lockdep splats will happen. but now using
> >>> rcuwait_wake_up() to wakeup grace-period kthread instead of wake_up(), in
> >>> rcuwait_wake_up() no spinlock will be acquired, so this commit remove using
> >>>
> >>> There are still spinlock-acquisition and spinlock-release invocations within the call path from rcuwait_wake_up().
> >>>
> >>> rcuwait_wake_up() -> wake_up_process() -> try_to_wake_up(), then:
> >>>
> >>> raw_spin_lock_irqsave()
> >>> ...
> >>> raw_spin_unlock_irqrestore
> >>
> >> Yes, but this is raw_spinlock acquisition and release(note: spinlock will convert to
> >> sleepable lock in Preempt-RT kernel, but raw spinlock is not change).
> >>
> >> acquire raw_spinlock -> acquire spinlock will trigger lockdep warning.
> >
> > Is this really safe in the long run though? I seem to remember there are
> > weird locking dependencies if RCU is used from within the scheduler [1].
> >
> > I prefer to keep it as irq_work_queue() unless you are seeing some benefit.
> > Generally, there has to be a 'win' or other justification for adding more
> > risk.
>
> On second thought, you are deleting a decent number of lines.
>
> What do others think?
>
> I will take a closer look later, I am interested in researching the new lock dependency this adds.
One place to start is rcu_read_unlock_trace_special(), keeping firmly
in mind that rcu_read_unlock_trace() is intended to be invoked from a
great many places.
Thanx, Paul
> - Joel
>
> >
> > thanks,
> >
> > - Joel
> > [1] http://www.joelfernandes.org/rcu/scheduler/locking/2019/09/02/rcu-schedlocks.html
> >
> >>> irq_work_queue(), invoke rcuwait_wake_up() directly in
> >>> call_rcu_tasks_generic().
> >>>
> >>> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> >>> ---
> >>> kernel/rcu/tasks.h | 16 +---------------
> >>> 1 file changed, 1 insertion(+), 15 deletions(-)
> >>>
> >>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index
> >>> baf7ec178155..757b8c6da1ad 100644
> >>> --- a/kernel/rcu/tasks.h
> >>> +++ b/kernel/rcu/tasks.h
> >>> @@ -39,7 +39,6 @@ struct rcu_tasks_percpu {
> >>> unsigned long rtp_jiffies;
> >>> unsigned long rtp_n_lock_retries;
> >>> struct work_struct rtp_work;
> >>> - struct irq_work rtp_irq_work;
> >>> struct rcu_head barrier_q_head;
> >>> struct list_head rtp_blkd_tasks;
> >>> int cpu;
> >>> @@ -112,12 +111,9 @@ struct rcu_tasks {
> >>> char *kname;
> >>> };
> >>>
> >>> -static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp);
> >>> -
> >>> #define DEFINE_RCU_TASKS(rt_name, gp, call, n)
> >>> \
> >>> static DEFINE_PER_CPU(struct rcu_tasks_percpu, rt_name ## __percpu) = {
> >>> \
> >>> .lock = __RAW_SPIN_LOCK_UNLOCKED(rt_name ##
> >>> __percpu.cbs_pcpu_lock), \
> >>> - .rtp_irq_work = IRQ_WORK_INIT_HARD(call_rcu_tasks_iw_wakeup),
> >>> \
> >>> };
> >>> \
> >>> static struct rcu_tasks rt_name =
> >>> \
> >>> {
> >>> \
> >>> @@ -273,16 +269,6 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
> >>> pr_info("%s: Setting shift to %d and lim to %d.\n", __func__,
> >>> data_race(rtp->percpu_enqueue_shift), data_race(rtp-
> >>>> percpu_enqueue_lim));
> >>> }
> >>>
> >>> -// IRQ-work handler that does deferred wakeup for call_rcu_tasks_generic().
> >>> -static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp) -{
> >>> - struct rcu_tasks *rtp;
> >>> - struct rcu_tasks_percpu *rtpcp = container_of(iwp, struct
> >>> rcu_tasks_percpu, rtp_irq_work);
> >>> -
> >>> - rtp = rtpcp->rtpp;
> >>> - rcuwait_wake_up(&rtp->cbs_wait);
> >>> -}
> >>> -
> >>> // Enqueue a callback for the specified flavor of Tasks RCU.
> >>> static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
> >>> struct rcu_tasks *rtp)
> >>> @@ -334,7 +320,7 @@ static void call_rcu_tasks_generic(struct rcu_head
> >>> *rhp, rcu_callback_t func,
> >>> rcu_read_unlock();
> >>> /* We can't create the thread unless interrupts are enabled. */
> >>> if (needwake && READ_ONCE(rtp->kthread_ptr))
> >>> - irq_work_queue(&rtpcp->rtp_irq_work);
> >>> + rcuwait_wake_up(&rtp->cbs_wait);
> >>> }
> >>>
> >>> // RCU callback function for rcu_barrier_tasks_generic().
> >>> --
> >>> 2.25.1
> >>
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