Re: [PATCH bpf-next v2 1/2] bpf, x64: Fix tailcall hierarchy

[Leon Hwang <hffilwlqm@gmail.com>]

On 2024/2/23 12:06, Pu Lehui wrote:
> 
> 
> On 2024/2/22 16:52, Leon Hwang wrote:

[SNIP]

>>   }
>>   @@ -575,6 +574,54 @@ static void emit_return(u8 **pprog, u8 *ip)
>>       *pprog = prog;
>>   }
>>   +DEFINE_PER_CPU(u32, bpf_tail_call_cnt);
> 
> Hi Leon, the solution is really simplifies complexity. If I understand
> correctly, this TAIL_CALL_CNT becomes the system global wise, not the
> prog global wise, but before it was limiting the TCC of entry prog.
> 

Correct. It becomes a PERCPU global variable.

But, I think this solution is not robust enough.

For example,

time      prog1           prog1
==================================>
line              prog2

this is a time-line on a CPU. If prog1 and prog2 have tailcalls to run,
prog2 will reset the tail_call_cnt on current CPU, which is used by
prog1. As a result, when the CPU schedules from prog2 to prog1,
tail_call_cnt on current CPU has been reset to 0, no matter whether
prog1 incremented it.

The tail_call_cnt reset issue happens too, even if PERCPU tail_call_cnt
moves to 'struct bpf_prog_aux', i.e. one kprobe bpf prog can be
triggered on many functions e.g. cilium/pwru. However, this moving is
better than this solution.

I think, my previous POC of 'struct bpf_prog_run_ctx' would be better.
I'll resend it later, with some improvements.

Thanks,
Leon