linux-kernel - Re: vfs-scale, chroot

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Message-ID: <AANLkTik-AdWpPqmR5TSCuijGwe3XLXm=HYVdUOhK3dDT@mail.gmail.com>
Date:	Thu, 13 Jan 2011 12:25:53 +1100
From:	Nick Piggin <npiggin@...il.com>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	"J. R. Okajima" <hooanon05@...oo.co.jp>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: vfs-scale, chroot

On Thu, Jan 13, 2011 at 7:04 AM, Trond Myklebust
<Trond.Myklebust@...app.com> wrote:

> BTW, Nick: Given that some filesystems such as NFS are _always_ going to
> reject LOOKUP_RCU,

That's not very optimistic of you... why is that a given, my I ask?
Or do you just mean as-in the current code?

> it would appear to be completely out of place to use
> the 'unlikely()' keyword when testing the results of path_walk_rcu() and
> friends. In particular when the kernel is running with nfsroot, we're
> saying that 100% of all cases are 'unlikely'...

Well that _is_ an accepted use of branch annotations. For example it
is used when scheduling realtime tasks, because even if some systems
will do 99.9% of their scheduling on realtime tasks, it is not the common
case.

I'm not saying that applies here, but: if path walk performance is
important, then we should use local caching and rcu-walk. If not, then
why do we care about slightly slower branch?

The annotations really help to reduce icache penalty of added
complexity which is why I like them, but I'm happy to remove them
where they don't make sense of course.

Thanks,
Nick
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