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

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Message-ID: <20181015153348.GB8952@tardis>
Date:   Mon, 15 Oct 2018 23:33:48 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 Mon, Oct 15, 2018 at 05:09:03PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

Ok, in that case, there should be no significant performance difference.

> 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?

Well, that commit is trying to avoid queue a work on an offlined CPU,
because according to workqueue API, it's the users' responsibility to
make sure the CPU is online when a work item enqueued. So there is a
difference ;-)

But I don't have any objection to revert it with your proposal, since
yours is more simple and straight-forward, and doesn't perform worse if
NUMA nodes and RCU nodes have one-to-one corresponding.

Besides, I think even if we observe some performance difference in the
future, the best way to solve that is to make workqueue have a more
fine-grained affine group than a NUMA node.

Regards,
Boqun

> 
> > Regards,
> > Boqun
> 
> Sebastian

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