On Tue, Oct 12, 2021 at 7:54 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 10/12/21 5:47 AM, Andrii Nakryiko wrote:
> > On Mon, Oct 11, 2021 at 3:57 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >> On 10/8/21 2:03 AM, andrii.nakryiko@xxxxxxxxx wrote:
> >>> From: Andrii Nakryiko <andrii@xxxxxxxxxx>
> >>>
> >>> Enhance existing selftests to demonstrate the use of custom
> >>> .data/.rodata sections.
> >>>
> >>> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> >>
> >> Just a thought, but wouldn't the actual demo / use case be better to show that we can
> >> now have a __read_mostly attribute which implies SEC(".data.read_mostly") section?
> >>
> >> Would be nice to add a ...
> >>
> >> #define __read_mostly SEC(".data.read_mostly")
> >>
> >> ... into tools/lib/bpf/bpf_helpers.h along with the series for use out of BPF programs
> >> as I think this should be a rather common use case. Thoughts?
> >
> > But what's so special about the ".data.read_mostly" ELF section for
> > BPF programs? It's just another data section with no extra semantics.
> > So unclear why we need to have a dedicated #define for that?..
>
> I mean semantics are implicit that only vars would be located there which are
> by far more read than written to. Placing into separate .data.read_mostly would
> help to reduce cache misses due to false sharing e.g. if they are otherwise placed
> near vars which are written more often (silly example would be some counter in
> the prog).
We've discussed this offline. We concluded that it's a bit premature
to add #define __read_mostly into bpf_helpers.h, but I'll use
__read_mostly as part of a selftests to demonstrate this concept.
>
> >>> ---
> >>> .../selftests/bpf/prog_tests/skeleton.c | 25 +++++++++++++++++++
> >>> .../selftests/bpf/progs/test_skeleton.c | 10 ++++++++
> >>> 2 files changed, 35 insertions(+)
> >> [...]
> >>> diff --git a/tools/testing/selftests/bpf/progs/test_skeleton.c b/tools/testing/selftests/bpf/progs/test_skeleton.c
> >>> index 441fa1c552c8..47a7e76866c4 100644
> >>> --- a/tools/testing/selftests/bpf/progs/test_skeleton.c
> >>> +++ b/tools/testing/selftests/bpf/progs/test_skeleton.c
> >>> @@ -40,9 +40,16 @@ int kern_ver = 0;
> >>>
> >>> struct s out5 = {};
> >>>
> >>> +const volatile int in_dynarr_sz SEC(".rodata.dyn");
> >>> +const volatile int in_dynarr[4] SEC(".rodata.dyn") = { -1, -2, -3, -4 };
> >>> +
> >>> +int out_dynarr[4] SEC(".data.dyn") = { 1, 2, 3, 4 };
> >>> +
> >>> SEC("raw_tp/sys_enter")
> >>> int handler(const void *ctx)
> >>> {
> >>> + int i;
> >>> +
> >>> out1 = in1;
> >>> out2 = in2;
> >>> out3 = in3;
> >>> @@ -53,6 +60,9 @@ int handler(const void *ctx)
> >>> bpf_syscall = CONFIG_BPF_SYSCALL;
> >>> kern_ver = LINUX_KERNEL_VERSION;
> >>>
> >>> + for (i = 0; i < in_dynarr_sz; i++)
> >>> + out_dynarr[i] = in_dynarr[i];
> >>> +
> >>> return 0;
> >>> }
> >>>
> >>>
> >>
