Re: "struct perf_sample_data" alignment

[Alexei Starovoitov]

On Fri, Mar 5, 2021 at 7:01 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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];

bpf side doesn't care about about cacheline aligned.
No need to add new typedef just for that.