On Tue, 2024-06-04 at 20:27 -0400, Steven Rostedt wrote: > 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 > Isn't this basically a perfect example of something where profile guided optimization should work? Thanks, Niklas
