userland process PID/TGID in the RPC layer

Dec. 20, 2024

      Hello everyone,

I am currently working on my bachelor thesis and I have a question regarding PIDs/TGIDs in the sunrpc layer of the kernel.
I was wondering if I can obtain the PID/TGID of the userland process that originally caused a RPC request to be transmitted, from within the RPC request itself.

Here is what I did:
With a BPF program I collect the following metrics from a tracepoint inside “xprt_request_transmit()” function defined in /net/sunrpc/xprt.c:

1. current task name 
2. current PID/TGID as a return value of bpf_get_current_pid_tgid()
3. The TGID assigned in the tk_owner field of struct rpc_task.  (Using: BPF_CORE_READ(rpc_rqst, rq_task, tk_owner))

From the sources I have learned that TGID from 3. is assigned  in “rpc_init_task() function” defined in /net/sunrpc/sched.c:

	task->tk_owner = current->tgid;

In the output of the BPF program I see records with the following entries for the above metrics:

1. "kworker/u389:3”
2. "PID": 455045, "TGID": 455045
3.  "rpc_tk_owner_tgid": 3989219

Here, evidently, the thread that executes the "xprt_request_transmit()” differs from the one that assigned the rpc_tk_owner_tgid in “rpc_init_task()”. 
The latter was executed by the userland process. 

But there are also records like this:

1. "kworker/u128:4"
2. "PID": 1457360, "TGID": 1457360
3.  "rpc_tk_owner_tgid": 1457360

Here, the kworker did both the jobs.

Now to my questions:

1. What determines which thread executes rpc_init_task()? 
2. How can I, with certainty, obtain the userland process’ PID/TGID from inside rpc_init_task() ?

Thank you for your answers.

Best regards,
Sandro

userland process PID/TGID in the RPC layer

sandro