On Wed, Mar 18, 2020 at 12:40:33PM -0700, Dan Williams wrote: > Yes, editorializing on unlikely(). Specifically I would normally ask > for perf numbers to show that the hint is worth it, but I talked > myself out of asking for that in this case. Ah, now I understand! Yes, it's just gilding the lily, but as long as I was messing about in the area it seemed worth adding. You're saying that if the code were a hotter code path, you'd want benchmarks, but given its actual infrequent usage, you're satisfied with the argument that it's probably right; it's not like we've crippled the kernel even if wrong. > I'm referring to this: > > if (is_shuffle_order(order)) > add_to_free_area_random(page, &zone->free_area[order], > > Where shuffle order is MAX_ORDER-1. I.e. this code is only triggered > when we might be releasing a 4MB buddy-page. Ding! Okay, now it makes sense. I didn't look that far up the call stack. I was auditing each and every call to a get_random function in the kernel, when I came across this call site, struggled to understand what it was doing, and rewrote it to be less wrong. I only vaguely understand where it fits into the larger mm/ system so I never noticed that condition on its use. Yes, given that infrequent use, I'm even happier that I focused on code size rather than performance. Thank you for taking the time to explain.
