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Message-ID: <ca1037935d570f70fd8900e18f1c7149298c5bf3.camel@linux.ibm.com>
Subject: Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator
From: Niklas Schnelle <schnelle@linux.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>, Andrew Lunn <andrew@lunn.ch>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Paolo Abeni <pabeni@redhat.com>,
        Mina
 Almasry <almasrymina@google.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-alpha@vger.kernel.org, linux-mips@vger.kernel.org,
        linux-parisc@vger.kernel.org, sparclinux@vger.kernel.org,
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        bpf@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org,
        "David S.
 Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>, Jakub
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        Donald Hunter <donald.hunter@gmail.com>,
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        Richard Henderson
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        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
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        Thomas Bogendoerfer
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        "James E.J. Bottomley"
 <James.Bottomley@hansenpartnership.com>,
        Helge Deller <deller@gmx.de>, Andreas Larsson <andreas@gaisler.com>,
        Jesper Dangaard Brouer
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        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Masami
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        Mathieu Desnoyers
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        Arnd Bergmann <arnd@arndb.de>, Alexei
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        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau
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        Eduard Zingerman <eddyz87@gmail.com>, Song Liu
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        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend
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        KP Singh <kpsingh@kernel.org>, Stanislav
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        Hao Luo <haoluo@google.com>, Jiri Olsa
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        Steffen Klassert <steffen.klassert@secunet.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        David Ahern
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        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Shuah Khan <shuah@kernel.org>, Sumit Semwal <sumit.semwal@linaro.org>,
        Christian =?ISO-8859-1?Q?K=F6nig?= <christian.koenig@amd.com>,
        Pavel
 Begunkov <asml.silence@gmail.com>, David Wei <dw@davidwei.uk>,
        Yunsheng Lin
 <linyunsheng@huawei.com>,
        Shailend Chand <shailend@google.com>,
        Harshitha
 Ramamurthy <hramamurthy@google.com>,
        Shakeel Butt
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        Jeroen de Borst <jeroendb@google.com>,
        Praveen
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        Willem de Bruijn
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        Kaiyuan Zhang <kaiyuanz@google.com>
Date: Fri, 07 Jun 2024 09:55:41 +0200
In-Reply-To: <20240604202738.3aab6308@gandalf.local.home>
References: <20240530201616.1316526-1-almasrymina@google.com>
	 <20240530201616.1316526-6-almasrymina@google.com>
	 <bea8b8bf1630309bb004f614e4a3c7f684a6acb6.camel@redhat.com>
	 <20240604121551.07192993@gandalf.local.home>
	 <20240604163158.GB21513@ziepe.ca>
	 <20240604124243.66203a46@gandalf.local.home>
	 <3be107ce-3d9f-4528-b9f7-1c9e38da0688@lunn.ch>
	 <20240604202738.3aab6308@gandalf.local.home>
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On Tue, 2024-06-04 at 20:27 -0400, Steven Rostedt wrote:
> On Wed, 5 Jun 2024 01:44:37 +0200
> Andrew Lunn <andrew@lunn.ch> wrote:
>=20
> > > Interesting, as I sped up the ftrace ring buffer by a substantial amo=
unt by
> > > adding strategic __always_inline, noinline, likely() and unlikely()
> > > throughout the code. It had to do with what was considered the fast p=
ath
> > > and slow path, and not actually the size of the function. gcc got it
> > > horribly wrong. =20
> >=20
> > And what did the compiler people say when you reported gcc was getting
> > it wrong?
> >=20
> > 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.
> >=20
>=20
> How is the compiler going to know which path is going to be taken the mos=
t?
> 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 on=
e
> 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.
>=20
> 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.
>=20
> How is a compiler going to know that?
>=20
> -- Steve
>=20

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

Thanks,
Niklas