linux-kernel - Re: [RFD] workqueue: WQ_MEM_RECLAIM usage in network drivers

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Message-ID: <20160318204623.GM20028@mtj.duckdns.org>
Date:	Fri, 18 Mar 2016 16:46:23 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Jeff Layton <jlayton@...chiereds.net>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Anna Schumaker <anna.schumaker@...app.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org,
	Amitoj Kaur Chawla <amitoj1606@...il.com>,
	kernel-team@...com, Johannes Weiner <hannes@...xchg.org>,
	Johannes Berg <johannes@...solutions.net>,
	Eva Rachel Retuya <eraretuya@...il.com>,
	Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
	linux-wireless@...r.kernel.org
Subject: Re: [RFD] workqueue: WQ_MEM_RECLAIM usage in network drivers

Hello, Jeff.

On Thu, Mar 17, 2016 at 09:32:16PM -0400, Jeff Layton wrote:
> > * Are network devices expected to be able to serve as a part of
> >   storage stack which is depended upon for memory reclamation?
> 
> I think they should be. Cached NFS pages can consume a lot of memory,
> and flushing them generally takes network device access.

But does that actually work?  It's pointless to add WQ_MEM_RECLAIM to
workqueues unless all other things are also guaranteed to make forward
progress regardless of memory pressure.

> > * If so, are all the pieces in place for that to work for all (or at
> >   least most) network devices?  If it's only for a subset of NICs, how
> >   can one tell whether a given driver needs forward progress guarantee
> >   or not?
> > 
> > * I assume that wireless drivers aren't and can't be used in this
> >   fashion.  Is that a correction assumption?
> > 
> 
> People do mount NFS over wireless interfaces. It's not terribly common
> though, in my experience.

Ditto, I'm very skeptical that this actually works in practice and
people expect and depend on it.  I don't follow wireless development
closely but haven't heard anyone talking about reserving memory pools
or people complaining about wireless being the cause of OOM.

So, I really want to avoid spraying WQ_MEM_RECLAIM if it doesn't serve
actual purposes.  It's wasteful, sets bad precedences and confuses
future readers.

Thanks.

-- 
tejun