linux-kernel - Re: [PATCH] rcu: Use cpus_read_lock() while looking at cpu_online

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Date:   Wed, 19 Sep 2018 13:55:21 -0700
From:   Tejun Heo <tj@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        tglx@...utronix.de, Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] rcu: Use cpus_read_lock() while looking at
 cpu_online_mask

Hello,

On Tue, Sep 11, 2018 at 10:02:22AM -0700, Paul E. McKenney wrote:
> > Doesn't work for me because it is still within the preempt-disable
> > section :/.
> > Would it work to use WORK_CPU_UNBOUND? As far as I understand it, the
> > CPU number does not matter, you just want to spread it across multiple
> > CPUs in the NUMA case.
> 
> Locality is a good thing, but yes, something like this?
> 
> 	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && /* or whatever it is called */
> 	    unlikely(cpu > rnp->grphi - rnp->grplo))
> 
> Another approach that might be better longer term would be to have a
> workqueue interface that treats the specified CPU as a suggestion,
> and silently switches to WORK_CPU_UNBOUND if there is any problem
> whatsoever with the specified CPU.  Tejun, Lai, thoughts?

Unbound workqueue is NUMA-affine by default, so using it by default
might not harm anything.  Also, per-cpu work items get unbound from
the cpu if the cpu goes down while the work item is running or queued,
so it might just work already.

Thanks.

-- 
tejun