linux-kernel - Re: [PATCH RCU -stable] Make need_resched() respond to urgent RCU-QS needs

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Date:   Thu, 29 Nov 2018 13:39:22 +0100
From:   Greg KH <greg@...ah.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     stable@...r.kernel.org, mhillenb@...zon.de, dwmw2@...radead.org,
        peterz@...radead.org, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RCU -stable] Make need_resched() respond to urgent RCU-QS
 needs

On Sun, Nov 04, 2018 at 06:34:34PM -0800, Paul E. McKenney wrote:
> commit 92aa39e9dc77 upstream.
> 
> The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
> need for an RCU quiescent state from the force-quiescent-state processing
> within the grace-period kthread to context switches and to cond_resched().
> Unfortunately, such urgent needs are not communicated to need_resched(),
> which is sometimes used to decide when to invoke cond_resched(), for
> but one example, within the KVM vcpu_run() function.  As of v4.15, this
> can result in synchronize_sched() being delayed by up to ten seconds,
> which can be problematic, to say nothing of annoying.
> 
> This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
> rcu_check_callbacks(), which is invoked from the scheduling-clock
> interrupt handler.  If the current task is not an idle task and is
> not executing in usermode, a context switch is forced, and either way,
> the rcu_dynticks.rcu_urgent_qs variable is set to false.  If the current
> task is an idle task, then RCU's dyntick-idle code will detect the
> quiescent state, so no further action is required.  Similarly, if the
> task is executing in usermode, other code in rcu_check_callbacks() and
> its called functions will report the corresponding quiescent state.
> 
> Reported-by: Marius Hillenbrand <mhillenb@...zon.de>
> Reported-by: David Woodhouse <dwmw2@...radead.org>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> [ paulmck: Backported to make patch apply cleanly on older versions. ]
> Tested-by: Marius Hillenbrand <mhillenb@...zon.de>
> Cc: <stable@...r.kernel.org> # 4.12.x - 4.19.x
> 

Now queued up, thanks.

greg k-h