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Date: Wed, 5 Jun 2024 02:52:29 +0200
From: Andrew Lunn <andrew@lunn.ch>
To: Steven Rostedt <rostedt@goodmis.org>
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Subject: Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator
Message-ID: <cbf3e752-8f7e-4e74-a709-8b812bdc36c0@lunn.ch>
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>
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> 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?

It might have some heuristics to try to guess unlikely/likely, but
that is not what we are talking about here.

How much difference did 'always_inline' and 'noinline' make? Hopefully
the likely is enough of a clue it should prefer to inline whatever is
in that branch, where as for the unlikely case it can do a function
call.

But compilers is not my thing, which is why i would reach out to the
compiler people and ask them, is it expected to get this wrong, could
it be made better?

   Andrew