netdev - Re: Ottawa and slow hash-table resize

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Message-ID: <20150224104839.GM3713@acer.localdomain>
Date:	Tue, 24 Feb 2015 10:48:39 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Thomas Graf <tgraf@...g.ch>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>,
	"josh@...htriplett.org" <josh@...htriplett.org>,
	"alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
	"herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
	"ying.xue@...driver.com" <ying.xue@...driver.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>
Subject: Re: Ottawa and slow hash-table resize

On 24.02, David Laight wrote:
> From: Patrick McHardy
> > On 24.02, Thomas Graf wrote:
> > > On 02/23/15 at 03:06pm, Paul E. McKenney wrote:
> > > > On Mon, Feb 23, 2015 at 05:32:52PM -0500, David Miller wrote:
> > > > > I just did a quick scan of all code paths that do inserts into an
> > > > > rhashtable, and it seems like all of them can easily block.  So why
> > > > > don't we do that?  Make inserts sleep on an rhashtable expansion
> > > > > waitq.
> > > > >
> > > > > There could even be a counter of pending inserts, so the expander can
> > > > > decide to expand further before waking the inserting threads up.
> > > >
> > > > Should be reasonably simple, and certainly seems worth a try!
> > >
> > > Agreed. Definitely desirable for nft_hash. I like the pending counter
> > > idea. I'm experimenting with various ideas on blocking inserts for
> > > Netlink. Blocking too long might open DoS vectors as one app could
> > > easily delay the creation of sockets for other applications.
> > 
> > Regarding nft_hash, blocking in the netlink path certainly seems fine,
> > but we will soon also have inserts from the packet processing path,
> > where we obviously can't block.
> 
> Why not an option to do a synchronous 'expand on insert' for
> codepaths that can block?

Sounds perfectly fine to me, just wanted to point out that we also need
to handle the non-synchronous case properly.
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