Hi Leon,
On Mon, Jan 13, 2025 at 11:24:35PM +0800, Leon Hwang wrote:
> This patch set introduces global per-CPU data, similar to commit
> 6316f78306c1 ("Merge branch 'support-global-data'"), to reduce restrictions
> in C for BPF programs.
>
> With this enhancement, it becomes possible to define and use global per-CPU
> variables, much like the DEFINE_PER_CPU() macro in the kernel[0].
>
> The idea stems from the bpflbr project[1], which itself was inspired by
> retsnoop[2]. During testing of bpflbr on the v6.6 kernel, two LBR
> (Last Branch Record) entries were observed related to the
> bpf_get_smp_processor_id() helper.
>
> Since commit 1ae6921009e5 ("bpf: inline bpf_get_smp_processor_id() helper"),
> the bpf_get_smp_processor_id() helper has been inlined on x86_64, reducing
> the overhead and consequently minimizing these two LBR records.
>
> However, the introduction of global per-CPU data offers a more robust
> solution. By leveraging the percpu_array map and percpu instructions,
> global per-CPU data can be implemented intrinsically.
>
> This feature also facilitates sharing per-CPU information between tail
> callers and callees or between freplace callers and callees through a
> shared global per-CPU variable. Previously, this was achieved using a
> 1-entry percpu map, which this patch set aims to improve upon.
I think this would be great to have. bpftrace would've liked to use this
for its recent big string support, but instead had to simulate a percpu
global through regular globals.
Thanks,
Daniel
