Re: return probe not executed on SMP system

[Hien Nguyen <hien at us dot ibm dot com>]

Guang Lei Li wrote:

> I applied your patch, and I also raise the maxaction from 1 to 10, then
> it worked really well, all return probed are caught.
>  
Do you mean maxactive? maxactive is not default to 1.

> But the default setting of maxaction to 1 is too low for a multi-processor
> system. It will cause a lot of misses of kretprobe(I counter it on Power5
> system for a while, about 2000 kprobes are caught, but only about 400
> kretprobe was caught, so about 1600 kretprobes are missed)
>
> I read the "kernel/kprobes.c", and found:
>
> /* Pre-allocate memory for max kretprobe instances */
> if (rp->maxactive <= 0) {
> #ifdef CONFIG_PREEMPT
>                rp->maxactive = max(10, 2 * NR_CPUS);
> #else
>                rp->maxactive = NR_CPUS;
> #endif
>        }
>  

By default maxactive is set to a value as in the #ifdef   #endif clause 
above

> Is it appropriate to just delete "if (rp->maxactive <= 0)", to change the 
> above block as:
>  
No, you should not make that change. When register a return probe, user 
has an option to change maxactive.
We need to implement the systemtap's interface to make that change in 
the future.

Hien.