Re: [RFC PATCH bpf-next] bpf: change syscall_nr type to int in struct syscall_tp_t

[Andrii Nakryiko]

On Thu, Oct 12, 2023 at 6:43 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Thu, 12 Oct 2023 13:45:50 +0200
> Artem Savkov <asavkov@xxxxxxxxxx> wrote:
>
> > linux-rt-devel tree contains a patch (b1773eac3f29c ("sched: Add support
> > for lazy preemption")) that adds an extra member to struct trace_entry.
> > This causes the offset of args field in struct trace_event_raw_sys_enter
> > be different from the one in struct syscall_trace_enter:
> >
> > struct trace_event_raw_sys_enter {
> >         struct trace_entry         ent;                  /*     0    12 */
> >
> >         /* XXX last struct has 3 bytes of padding */
> >         /* XXX 4 bytes hole, try to pack */
> >
> >         long int                   id;                   /*    16     8 */
> >         long unsigned int          args[6];              /*    24    48 */
> >         /* --- cacheline 1 boundary (64 bytes) was 8 bytes ago --- */
> >         char                       __data[];             /*    72     0 */
> >
> >         /* size: 72, cachelines: 2, members: 4 */
> >         /* sum members: 68, holes: 1, sum holes: 4 */
> >         /* paddings: 1, sum paddings: 3 */
> >         /* last cacheline: 8 bytes */
> > };
> >
> > struct syscall_trace_enter {
> >         struct trace_entry         ent;                  /*     0    12 */
> >
> >         /* XXX last struct has 3 bytes of padding */
> >
> >         int                        nr;                   /*    12     4 */
> >         long unsigned int          args[];               /*    16     0 */
> >
> >         /* size: 16, cachelines: 1, members: 3 */
> >         /* paddings: 1, sum paddings: 3 */
> >         /* last cacheline: 16 bytes */
> > };
> >
> > This, in turn, causes perf_event_set_bpf_prog() fail while running bpf
> > test_profiler testcase because max_ctx_offset is calculated based on the
> > former struct, while off on the latter:
> >
> >   10488         if (is_tracepoint || is_syscall_tp) {
> >   10489                 int off = trace_event_get_offsets(event->tp_event);
> >   10490
> >   10491                 if (prog->aux->max_ctx_offset > off)
> >   10492                         return -EACCES;
> >   10493         }
> >
> > What bpf program is actually getting is a pointer to struct
> > syscall_tp_t, defined in kernel/trace/trace_syscalls.c. This patch fixes
> > the problem by aligning struct syscall_tp_t with with struct
> > syscall_trace_(enter|exit) and changing the tests to use these structs
> > to dereference context.
> >
> > Signed-off-by: Artem Savkov <asavkov@xxxxxxxxxx>
>

I think these changes make sense regardless, can you please resend the
patch without RFC tag so that our CI can run tests for it?

> Thanks for doing a proper fix.
>
> Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

But looking at [0] and briefly reading some of the discussions you,
Steven, had. I'm just wondering if it would be best to avoid
increasing struct trace_entry altogether? It seems like preempt_count
is actually a 4-bit field in trace context, so it doesn't seem like we
really need to allocate an entire byte for both preempt_count and
preempt_lazy_count. Why can't we just combine them and not waste 8
extra bytes for each trace event in a ring buffer?

  [0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?id=b1773eac3f29cbdcdfd16e0339f1a164066e9f71

>
> -- Steve