On Mon, Aug 30, 2021 at 02:41:05PM -0700, Song Liu wrote:
> @@ -564,6 +565,18 @@ static void notrace inc_misses_counter(struct bpf_prog *prog)
> u64 notrace __bpf_prog_enter(struct bpf_prog *prog)
> __acquires(RCU)
> {
preempt_disable_notrace();
> +#ifdef CONFIG_PERF_EVENTS
> + /* Calling migrate_disable costs two entries in the LBR. To save
> + * some entries, we call perf_snapshot_branch_stack before
> + * migrate_disable to save some entries. This is OK because we
> + * care about the branch trace before entering the BPF program.
> + * If migrate happens exactly here, there isn't much we can do to
> + * preserve the data.
> + */
> + if (prog->call_get_branch)
> + static_call(perf_snapshot_branch_stack)(
> + this_cpu_ptr(&bpf_perf_branch_snapshot));
Here the comment is accurate, but if you recall the calling context
requirements of perf_snapshot_branch_stack from the last patch, you'll
see it requires you have at the very least preemption disabled, which
you just violated.
I think you'll find that (on x86 at least) the suggested
preempt_disable_notrace() incurs no additional branches.
Still, there is the next point to consider...
> +#endif
> rcu_read_lock();
> migrate_disable();
preempt_enable_notrace();
> if (unlikely(__this_cpu_inc_return(*(prog->active)) != 1)) {
> @@ -1863,9 +1892,23 @@ void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp)
> preempt_enable();
> }
>
> +DEFINE_PER_CPU(struct perf_branch_snapshot, bpf_perf_branch_snapshot);
> +
> static __always_inline
> void __bpf_trace_run(struct bpf_prog *prog, u64 *args)
> {
> +#ifdef CONFIG_PERF_EVENTS
> + /* Calling migrate_disable costs two entries in the LBR. To save
> + * some entries, we call perf_snapshot_branch_stack before
> + * migrate_disable to save some entries. This is OK because we
> + * care about the branch trace before entering the BPF program.
> + * If migrate happens exactly here, there isn't much we can do to
> + * preserve the data.
> + */
> + if (prog->call_get_branch)
> + static_call(perf_snapshot_branch_stack)(
> + this_cpu_ptr(&bpf_perf_branch_snapshot));
> +#endif
> cant_sleep();
In the face of ^^^^^^ the comment makes no sense. Still, what are the
nesting rules for __bpf_trace_run() and __bpf_prog_enter() ? I'm
thinking the trace one can nest inside an occurence of prog, at which
point you have pieces.
> rcu_read_lock();
> (void) bpf_prog_run(prog, args);
