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Message-ID: <CALm+0cWeYcp4jKV2H=g5MOHOg6WJ7se0S1g6BhZrwPACiFWYDA@mail.gmail.com>
Date: Sun, 17 Mar 2024 15:12:59 +0800
From: Z qiang <qiang.zhang1211@...il.com>
To: paulmck@...nel.org
Cc: frederic@...nel.org, joel@...lfernandes.org, neeraj.upadhyay@...nel.org,
rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu-tasks: Avoid rtp_irq_work triggering when the
rcu-tasks GP is ongoing
>
> On Mon, Mar 11, 2024 at 11:55:02AM +0800, Zqiang wrote:
> > This commit generate rcu_task_gp_in_progress() to check whether
> > the rcu-tasks GP is ongoing, if is ongoing, avoid trigger
> > rtp_irq_work to wakeup rcu tasks kthreads in call_rcu_tasks_generic().
> >
> > The test results are as follows:
> >
> > echo call_rcu_tasks_iw_wakeup > /sys/kernel/debug/tracing/set_ftrace_filter
> > echo 1 > /sys/kernel/debug/tracing/function_profile_enabled
> > insmod rcutorture.ko torture_type=tasks-tracing fwd_progress=4
> > sleep 600
> > rmmod rcutorture.ko
> > echo 0 > /sys/kernel/debug/tracing/function_profile_enabled
> > echo > /sys/kernel/debug/tracing/set_ftrace_filter
> >
> > head /sys/kernel/debug/tracing/trace_stat/function*
> >
> > original: 56376 apply patch: 33521
> >
> > Signed-off-by: Zqiang <qiang.zhang1211@...il.com>
>
> Note that rcu_seq_current() does not provide ordering. So are you
> sure that this change is safe on weakly ordered systems such as ARM?
The rcu_seq_current() provide implicit address-dependency barriers, or
did I miss something?
>
> For example, consider the following sequence of events:
>
> o The call_rcu_tasks_generic() function picks up the grace-period
> sequence number, which shows that there is a grace period in
> progress.
The rcu-callback has been enqueued to list before we pick up the
gp seq number.
>
> o The grace period ends, and sees no reason to start a new grace
> period.
the gp ends, the rcu_tasks_need_gpcb() will be invoked to check
whether to start a new gp. find pending callback, the new gp
will start or did I miss something?
Thanks
Zqiang
>
> o The call_rcu_tasks_generic() function sees no reason to wake
> up the grace-period kthread. There are no more calls to
> call_rcu_tasks*(), so the callback is never invoked.
>
> Or is there something that prevents this sequence of events from
> ever happening on weakly ordered systems?
>
> Thanx, Paul
>
> > ---
> >
> > original:
> > ==> /sys/kernel/debug/tracing/trace_stat/function0 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 13217 19292.52 us 1.459 us 8.834 us
> >
> > ==> /sys/kernel/debug/tracing/trace_stat/function1 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 15146 22377.01 us 1.477 us 22.873 us
> >
> > ==> /sys/kernel/debug/tracing/trace_stat/function2 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 12561 18125.76 us 1.443 us 6.372 us
> >
> > ==> /sys/kernel/debug/tracing/trace_stat/function3 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 15452 21770.57 us 1.408 us 6.710 us
> >
> > apply patch:
> > ==> /sys/kernel/debug/tracing/trace_stat/function0 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 8334 15121.13 us 1.814 us 4.457 us
> >
> > ==> /sys/kernel/debug/tracing/trace_stat/function1 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 8355 15760.51 us 1.886 us 14.775 us
> >
> > ==> /sys/kernel/debug/tracing/trace_stat/function2 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 7219 14194.27 us 1.966 us 42.440 us
> >
> > ==> /sys/kernel/debug/tracing/trace_stat/function3 <==
> > Function Hit Time Avg s^2
> > -------- --- ---- --- ---
> > call_rcu_tasks_iw_wakeup 9613 19850.04 us 2.064 us 91.023 us
> >
> > kernel/rcu/tasks.h | 8 +++++++-
> > 1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > index 147b5945d67a..36c7e1d441d0 100644
> > --- a/kernel/rcu/tasks.h
> > +++ b/kernel/rcu/tasks.h
> > @@ -317,6 +317,11 @@ static void call_rcu_tasks_iw_wakeup(struct irq_work *iwp)
> > rcuwait_wake_up(&rtp->cbs_wait);
> > }
> >
> > +static int rcu_task_gp_in_progress(struct rcu_tasks *rtp)
> > +{
> > + return rcu_seq_state(rcu_seq_current(&rtp->tasks_gp_seq));
> > +}
> > +
> > // 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)
> > @@ -375,7 +380,8 @@ 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))
> > + if (needwake && READ_ONCE(rtp->kthread_ptr) &&
> > + !rcu_task_gp_in_progress(rtp))
> > irq_work_queue(&rtpcp->rtp_irq_work);
> > }
> >
> > --
> > 2.17.1
> >
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