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

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Date:   Thu, 17 Jun 2021 22:04:34 +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 Wed, 16 Jun 2021 11:27:11 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
>> On Tue, 15 Jun 2021 21:03:51 -0400
>> Steven Rostedt <rostedt@...dmis.org> wrote:
>> 
>> > On Wed, 16 Jun 2021 09:46:22 +0900
>> > Masami Hiramatsu <mhiramat@...nel.org> 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