Re: [PATCH v3 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot

[Peter Zijlstra <peterz@infradead.org>]

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);