netdev - RE: [PATCH net] rhashtable: use cond

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CAEEEDF@AcuExch.aculab.com>
Date:	Thu, 26 Feb 2015 16:03:38 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Patrick McHardy' <kaber@...sh.net>
CC:	'Eric Dumazet' <eric.dumazet@...il.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"tgraf@...g.ch" <tgraf@...g.ch>,
	"pablo@...filter.org" <pablo@...filter.org>,
	"johunt@...mai.com" <johunt@...mai.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net] rhashtable: use cond_resched()

From: Patrick McHardy [mailto:kaber@...sh.net]
> On 26.02, David Laight wrote:
> > From: Eric Dumazet
> > Sent: 26 February 2015 15:21
> > > If a hash table has 128 slots and 16384 elems, expand to 256 slots
> > > takes more than one second. For larger sets, a soft lockup is detected.
> >
> > What on earth is it doing?
> > Presumably something to do with the rcu actions needed to allow
> > lockless lookup during resize.
> >
> > There has to be a better solution?
> > Perhaps even two sets of chain pointers down the hash lists.
> > Then the old hash table can be kept completely valid while the
> > whole 'unzip' action is done.
> 
> One of the main points of rhashtable is that you don't need those.

Would it be possible to keep the hash chains sorted into 'raw hash' order?
I think that would mean the 'unzip' only had to change a single linkage.

	David

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