Re: [PATCH 2/2] trace/kprobe: Remove limit on kretprobe maxactive

[Naveen N. Rao]

Masami Hiramatsu wrote:
> On Wed, 16 Jun 2021 11:27:11 +0900
> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> > On Tue, 15 Jun 2021 21:03:51 -0400
> > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > > On Wed, 16 Jun 2021 09:46:22 +0900
> > > Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > > 
> > > > To avoid such trouble, I had set the 4096 limitation for the maxactive
> > > > parameter. Of course 4096 may not enough for some use-cases. I'm welcome
> > > > to expand it (e.g. 32k, isn't it enough?), but removing the limitation
> > > > may cause OOM trouble easily.
> > > 
> > > What if you just made the max as 10 * number of possible cpus, or 4096,
> > > which ever is greater? Why would a user need more?
> >
> > It could be. But actually, that is not correct number because the
> > number of instances depends on the number of processes and the possiblity
> > of recursive. Thus the huge system which runs more than 64k processes,
> > may need more than that.
> >
> > > I'd still like to get a wrapper around function graph tracing so that
> > > kretprobes could use it. I think that would get rid of the requirement
> > > of maxactive, because isn't that just used to have a way to know the
> > > original return value?
> >
> > Hmm, yes, on some arch, it can be done. But on other arch we still need
> > current implementation for generic solution.
> > What I need is not fully wrapped by the function graph, but just share
> > the per-task (software) shadow stack.
>
> BTW, I have 2 ideas to fix this except for wrapper.
>
> 1. Use func-graph tracer API directly from dynamic event instead of
>   kretprobes. This will be enabled only if the arch supports fgraph
>   tracer and enable it. maxactive will be ignored if this is enabled,
>   and tracefs user may not need except for the return value 
>   (BTW, is that possible to access the stack? In some case, return
>   value can be passed via stack)
>
> 2. Move the kretprobe instance pool from kretprobe to struct task.
>   This pool will allocates one page per task, and shared among all
>   kretprobes. This pool will be allocated when the 1st kretprobe
>   is registered. maxactive will be kept for someone who wants to
>   use per-instance data. But since dynamic event doesn't use it,
>   it will be removed from tracefs and perf.

Won't this result in _more_ memory usage compared to what we have now?

Thanks,
Naveen