ftrace - set filter for finish_task_switch failed. error code -22

[Hemingjian]

I successfully build on kernel 5.15.30-051530-generic but it fails on insod module. dmesg when checking the info it says
[49232.363857] ftrace - set filter for finish_task_switch failed. error code -22

[Hemingjian]

I also try Linux hmj-PowerEdge-R730 6.2.0-20-generic #20-Ubuntu. But still the same problem

[jcsaezal]

Hi @Hemingjian

After looking into this issue, it seems that recent versions of GCC do inline the finish_task_switch() function. As a result, the Ftrace hook that PMCTrack attempts to install to be aware of invocations of that function fails to install, giving rise to the error you are experiencing.

We will try to find a consistent fix to the problem that works for multiple kernel versions. In the meantime, a quick workaround is to install a modified kernel where the following patch is applied (it tells GCC not to inline that specific function):

diff --git a/kernel/sched/core.c b/kernel/sched/core.c                                                                           
index 2a4918a1faa9..44e2333d5701 100644                                                                                                 
--- a/kernel/sched/core.c                                                                                                        
+++ b/kernel/sched/core.c                                                                                                               
@@ -5129,7 +5129,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,                                               
  * past. prev == current is still correct but we need to recalculate this_rq                                                           
  * because prev may have moved to another CPU.                                                                                  
  */                                                                                                                                    
-static struct rq *finish_task_switch(struct task_struct *prev)                                                                  
+noinline struct rq* finish_task_switch(struct task_struct *prev)                                                                       
        __releases(rq->lock)                                                                                                     
 {                                                                                                                                      
        struct rq *rq = this_rq(); 

This is currently working for us in various kernels, including those versions you mentioned. I hope this helps.