Re: "struct perf_sample_data" alignment

[Peter Zijlstra]

On Fri, Mar 05, 2021 at 09:36:30AM +0100, Peter Zijlstra wrote:
> 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.

This seems to be it... (completely untested)

---
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 3f7f89ea5e51..918a296d2ca2 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1032,7 +1032,9 @@ struct perf_sample_data {
 	u64				cgroup;
 	u64				data_page_size;
 	u64				code_page_size;
-} ____cacheline_aligned;
+};
+
+typedef struct perf_sample_data perf_sample_data_t ____cacheline_aligned;
 
 /* default value for data source */
 #define PERF_MEM_NA (PERF_MEM_S(OP, NA)   |\
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b0c45d923f0f..f32c623abef6 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -923,7 +923,7 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
  * bpf_perf_event_output
  */
 struct bpf_trace_sample_data {
-	struct perf_sample_data sds[3];
+	perf_sample_data_t sds[3];
 };
 
 static DEFINE_PER_CPU(struct bpf_trace_sample_data, bpf_trace_sds);