On Thu, Mar 04, 2021 at 07:45:44PM -0800, Linus Torvalds wrote: > That ____cacheline_aligned goes back many years, this is not new, it > seems to come from back in 2014: commit 2565711fb7d7 ("perf: Improve > the perf_sample_data struct layout").
long time ago... > But it really seems entirely and utterly bogus. That cacheline > alignment makes things *worse*, when the variables are on the local > stack. The local stack is already going to be dirty and in the cache, > and aligning those things isn't going to - and I quote from the code > in that commend in that commit - "minimize the cachelines touched". > > Quite the reverse. It's just going to make the stack frame use *more* > memory, and make any cacheline usage _worse_. IIRC there is more history here, but I can't seem to find references just now. What I remember is that since perf_sample_data is fairly large, unconditionally initializing the whole thing is *slow* (and -fauto-var-init=zero will hurt here). So at some point I removed that full initialization and made sure we only unconditionally touched the first few variables, which gave a measurable speedup. Then things got messy again and the commit 2565711fb7d7 referenced above was cleanup, to get back to that initial state. Now, you're right that __cacheline_aligned on on-stack (and this is indeed mostly on-stack) is fairly tedious (there were a few patches recently to reduce the amount of on-stack instances). I'll put it on the todo list, along with that hotplug stuff (which I tried to fix but ended up with an even bigger mess). I suppose we can try and not have the alignment for the on-stack instances while preserving it for the few off-stack ones. Also; we're running on the NMI stack, and that's not typically hot.