From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: rcu@vger.kernel.org, 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,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH tip/core/rcu 2/2] rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()
Date: Wed, 3 Apr 2019 11:50:46 +0200 [thread overview]
Message-ID: <20190403095046.GD4038@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190402131853.GV4102@linux.ibm.com>
On Tue, Apr 02, 2019 at 06:18:53AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 02, 2019 at 09:09:53AM +0200, Peter Zijlstra wrote:
> > On Mon, Apr 01, 2019 at 10:22:57AM -0700, Paul E. McKenney wrote:
> > > Or am I missing something that gets the scheduler on the job faster?
> >
> > Oh urgh, yah. So normally we only twiddle with the need_resched state:
> >
> > - while preempt_disabl(), such that preempt_enable() will reschedule
> > - from interrupt context, such that interrupt return will reschedule
> >
> > But the usage here 'violates' those rules and then there is an
> > unspecified latency between setting the state and it getting observed,
> > but no longer than 1 tick I would think.
>
> In general, yes, which is fine (famous last words) for normal grace
> periods but not so good for expedited grace periods.
>
> > I don't think we can go NOHZ with need_resched set, because the moment
> > we hit the idle loop with that set, we _will_ reschedule.
>
> Agreed, and I believe that transitioning to usermode execution also
> gives the scheduler a chance to take action.
>
> The one exception to this is when a nohz_full CPU running in nohz_full
> mode does a system call that decides to execute for a very long time.
> Last I checked, the scheduling-clock interrupt did -not- get retriggered
> in this case, and the delay could be indefinite, as in bad even for
> normal grace periods.
Right, there is that.
> > So in that respect the irq_work suggestion I made would fix things
> > properly.
>
> But wouldn't the current use of set_tsk_need_resched(current) followed by
> set_preempt_need_resched() work just as well in that case? The scheduler
> would react to these at the next scheduler-clock interrupt on their
> own, right? Or am I being scheduler-naive again?
Well, you have that unspecified delay. By forcing the (self) interrupt
you enforce a timely response.
next prev parent reply other threads:[~2019-04-03 9:51 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-29 18:26 [PATCH RFC tip/core/rcu 0/2] Real-time elimination of RCU_SOFTIRQ Paul E. McKenney
2019-03-29 18:26 ` [PATCH tip/core/rcu 1/2] rcu: Enable elimination of Tree-RCU softirq processing Paul E. McKenney
2019-03-29 18:26 ` [PATCH tip/core/rcu 2/2] rcu: Check for wakeup-safe conditions in rcu_read_unlock_special() Paul E. McKenney
2019-04-01 8:32 ` Peter Zijlstra
2019-04-01 17:22 ` Paul E. McKenney
2019-04-01 19:03 ` Paul E. McKenney
2019-04-02 7:09 ` Peter Zijlstra
2019-04-02 13:18 ` Paul E. McKenney
2019-04-03 9:50 ` Peter Zijlstra [this message]
2019-04-03 16:25 ` Paul E. McKenney
2019-04-04 19:49 ` Paul E. McKenney
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