linux-kernel - Re: [PATCH tip/core/rcu 02/15] rcu: Use timer as backstop for NOCB deferred wakeups

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Message-ID: <20170725141220.4d2d968e@vmware.local.home>
Date:   Tue, 25 Jul 2017 14:12:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com
Subject: Re: [PATCH tip/core/rcu 02/15] rcu: Use timer as backstop for NOCB
 deferred wakeups

On Mon, 24 Jul 2017 14:44:31 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> The handling of RCU's no-CBs CPUs has a maintenance headache, namely
> that if call_rcu() is invoked with interrupts disabled, the rcuo kthread
> wakeup must be defered to a point where we can be sure that scheduler
> locks are not held.  Of course, there are a lot of code paths leading
> from an interrupts-disabled invocation of call_rcu(), and missing any
> one of these can result in excessive callback-invocation latency, and
> potentially even system hangs.

What about using irq_work? That's what perf and ftrace use for such a
case.

-- Steve

> 
> This commit therefore uses a timer to guarantee that the wakeup will
> eventually occur.  If one of the deferred-wakeup points kicks in, then
> the timer is simply cancelled.
> 
> This commit also fixes up an incomplete removal of commits that were
> intended to plug remaining exit paths, which should have the added
> benefit of reducing the overhead of RCU's context-switch hooks.  In
> addition, it simplifies leader-to-follower callback-list handoff by
> introducing locking.  The call_rcu()-to-leader handoff continues to
> use atomic operations in order to maintain good real-time latency for
> common-case use of call_rcu().
>