From: David Woodhouse <dwmw2@infradead.org>
To: Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>,
Eric Dumazet <eric.dumazet@gmail.com>,
Paul Turner <pjt@google.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next v2 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin
Date: Fri, 07 Dec 2018 21:46:37 +0000 [thread overview]
Message-ID: <2c80b9d34540dad337ed2fce6f9d16233105a2c2.camel@infradead.org> (raw)
In-Reply-To: <f692efbbc9df09acb5f18ad16907e28cda520e4d.camel@redhat.com>
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On Fri, 2018-12-07 at 21:46 +0100, Paolo Abeni wrote:
> > I wonder if we can declare the common case functions as 'weak' so that
> > the link failures don't happen when they're absent.
>
> I experimented a previous version with alias. I avoided weak alias
> usage, because I [mis?]understood not all compilers have a complete
> support for them (e.g. clang).
> Also, with weak ref, a coding error that is now discovered at build
> time will result in worse performance at runtime, likely with some
> uncommon configuration, possibly not as easily detected. I'm unsure
> that would be better ?!?
I think everything supports weak linkage; we've been using it for
years.
> > Once we extend this past the network code, especially to file systems'
> > f_ops, I suspect we're going to want to use something like static keys
> > to patch the common cases at runtime — perhaps changing the f_ops
> > default according to what the root file system is, etc.
>
> I'm sorry, I don't follow here. I think static keys can't be used for
> the reported network case: we have different list elements each
> contaning a different function pointer and we access/use
> different ptr on a per packet basis.
Yes, the alternatives would be used to change the "likely" case.
We still do the "if (fn == default_fn) default_fn(); else (*fn)();"
part; or even the variant with two (or more) common cases.
It's just that the value of 'default_fn' can be changed at runtime
(with patching like alternatives/static keys, since of course it has to
be a direct call).
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next prev parent reply other threads:[~2018-12-07 21:46 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-05 18:13 [PATCH net-next v2 0/4] net: mitigate retpoline overhead Paolo Abeni
2018-12-05 18:13 ` [PATCH net-next v2 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin Paolo Abeni
2018-12-07 9:46 ` David Woodhouse
2018-12-07 20:46 ` Paolo Abeni
2018-12-07 21:46 ` David Woodhouse [this message]
2018-12-11 22:28 ` Paolo Abeni
2018-12-05 18:13 ` [PATCH net-next v2 2/4] net: use indirect call wrappers at GRO network layer Paolo Abeni
2018-12-05 18:13 ` [PATCH net-next v2 3/4] net: use indirect call wrappers at GRO transport layer Paolo Abeni
2018-12-21 2:57 ` Stephen Hemminger
2018-12-05 18:13 ` [PATCH net-next v2 4/4] udp: use indirect call wrappers for GRO socket lookup Paolo Abeni
2018-12-06 4:49 ` [PATCH net-next v2 0/4] net: mitigate retpoline overhead David Miller
2018-12-07 6:24 ` David Miller
2018-12-07 6:28 ` David Miller
2018-12-07 20:29 ` Paolo Abeni
2018-12-07 21:06 ` David Miller
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