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Message-ID: <ca1037935d570f70fd8900e18f1c7149298c5bf3.camel@linux.ibm.com>
Date: Fri, 07 Jun 2024 09:55:41 +0200
From: Niklas Schnelle <schnelle@...ux.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>, Andrew Lunn <andrew@...n.ch>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Paolo Abeni <pabeni@...hat.com>,
        Mina
 Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        "David S.
 Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub
 Kicinski <kuba@...nel.org>,
        Donald Hunter <donald.hunter@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Richard Henderson
 <richard.henderson@...aro.org>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        Thomas Bogendoerfer
 <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley"
 <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>, Andreas Larsson <andreas@...sler.com>,
        Jesper Dangaard Brouer
 <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Masami
 Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers
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        Arnd Bergmann <arnd@...db.de>, Alexei
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        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau
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        Eduard Zingerman <eddyz87@...il.com>, Song Liu
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        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend
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        KP Singh <kpsingh@...nel.org>, Stanislav
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        Hao Luo <haoluo@...gle.com>, Jiri Olsa
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        Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        David Ahern
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        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Shuah Khan <shuah@...nel.org>, Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Pavel
 Begunkov <asml.silence@...il.com>, David Wei <dw@...idwei.uk>,
        Yunsheng Lin
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        Shailend Chand <shailend@...gle.com>,
        Harshitha
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        Shakeel Butt
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        Jeroen de Borst <jeroendb@...gle.com>,
        Praveen
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        Willem de Bruijn
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        Kaiyuan Zhang <kaiyuanz@...gle.com>
Subject: Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator

On Tue, 2024-06-04 at 20:27 -0400, Steven Rostedt wrote:
> On Wed, 5 Jun 2024 01:44:37 +0200
> Andrew Lunn <andrew@...n.ch> wrote:
> 
> > > Interesting, as I sped up the ftrace ring buffer by a substantial amount by
> > > adding strategic __always_inline, noinline, likely() and unlikely()
> > > throughout the code. It had to do with what was considered the fast path
> > > and slow path, and not actually the size of the function. gcc got it
> > > horribly wrong.  
> > 
> > And what did the compiler people say when you reported gcc was getting
> > it wrong?
> > 
> > Our assumption is, the compiler is better than a human at deciding
> > this. Or at least, a human who does not spend a long time profiling
> > and tuning. If this assumption is not true, we probably should be
> > trying to figure out why, and improving the compiler when
> > possible. That will benefit everybody.
> > 
> 
> How is the compiler going to know which path is going to be taken the most?
> There's two main paths in the ring buffer logic. One when an event stays on
> the sub-buffer, the other when the event crosses over to a new sub buffer.
> As there's 100s of events that happen on the same sub-buffer for every one
> time there's a cross over, I optimized the paths that stayed on the
> sub-buffer, which caused the time for those events to go from 250ns down to
> 150 ns!. That's a 40% speed up.
> 
> I added the unlikely/likely and 'always_inline' and 'noinline' paths to
> make sure the "staying on the buffer" path was always the hot path, and
> keeping it tight in cache.
> 
> How is a compiler going to know that?
> 
> -- Steve
> 

Isn't this basically a perfect example of something where profile
guided optimization should work?

Thanks,
Niklas

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