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

[Naveen N. Rao]

Masami Hiramatsu wrote:
> On Mon, 14 Jun 2021 23:33:29 +0530
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
>
> > We currently limit maxactive for a kretprobe to 4096 when registering
> > the same through tracefs. The comment indicates that this is done so as
> > to keep list traversal reasonable. However, we don't ever iterate over
> > all kretprobe_instance structures. The core kprobes infrastructure also
> > imposes no such limitation.
> >
> > Remove the limit from the tracefs interface. This limit is easy to hit
> > on large cpu machines when tracing functions that can sleep.
> >
> > Reported-by: Anton Blanchard <anton@xxxxxxxxxx>
> > Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
>
> OK, but I don't like to just remove the limit (since it can cause
> memory shortage easily.)
> Can't we make it configurable? I don't mean Kconfig, but 
> tracefs/options/kretprobe_maxactive, or kprobes's debugfs knob.
>
> Hmm, maybe debugfs/kprobes/kretprobe_maxactive will be better since
> it can limit both trace_kprobe and kprobes itself.

I don't think it is good to put a new tunable in debugfs -- we don't 
have any kprobes tunable there, so this adds a dependency on debugfs 
which shouldn't be necessary.

/proc/sys/debug/ may be a better fit since we have the 
kprobes-optimization flag to disable optprobes there, though I'm not 
sure if a new sysfs file is agreeable.


But, I'm not too sure this really is a problem. Maxactive is a user 
_opt-in_ feature which needs to be explicitly added to an event 
definition. In that sense, isn't this already a tunable?


- Naveen