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

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Date:   Tue, 15 Jun 2021 23:11:27 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Anton Blanchard <anton@...abs.org>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 2/2] trace/kprobe: Remove limit on kretprobe maxactive

Masami Hiramatsu wrote:
> On Mon, 14 Jun 2021 23:33:29 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> 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@...abs.org>
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
> 
> 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