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

[Steven Rostedt]

On Wed, 5 Jun 2024 01:44:37 +0200
Andrew Lunn <andrew@xxxxxxx> 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