netdev - Re: [PATCH net-next v10 05/14] netdev: netdevice devmem allocator

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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
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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