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

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Date:   Mon, 15 Oct 2018 17:09:03 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
        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

On 2018-10-15 23:07:15 [+0800], Boqun Feng wrote:
> Hi, Sebastian
Hi Boqun,

> On Mon, Oct 15, 2018 at 04:42:17PM +0200, Sebastian Andrzej Siewior wrote:
> > On 2018-10-13 06:48:13 [-0700], Paul E. McKenney wrote:
> > > 
> > > My concern would be that it would queue it by default for the current
> > > CPU, which would serialize the processing, losing the concurrency of
> > > grace-period initialization.  But that was a long time ago, and perhaps
> > > workqueues have changed. 
> > 
> > but the code here is always using the first CPU of a NUMA node or did I
> > miss something?
> > 
> 
> The thing is the original way is to pick one CPU for a *RCU* node to
> run the grace-period work, but with your proposal, if a RCU node is
> smaller than a NUMA node (having fewer CPUs), we could end up having two
> grace-period works running on one CPU. I think that's Paul's concern.

Ah. Okay. From what I observed, the RCU nodes and NUMA nodes were 1:1
here. Noted.
Given that I can enqueue a work item on an offlined CPU I don't see why
commit fcc6354365015 ("rcu: Make expedited GPs handle CPU 0 being
offline") should make a difference. Any objections to just revert it?

> Regards,
> Boqun

Sebastian