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

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Date:   Fri, 18 Jun 2021 14:11:12 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Anton Blanchard <anton@...abs.org>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/2] trace/kprobe: Remove limit on kretprobe maxactive

Masami Hiramatsu wrote:
> On Thu, 17 Jun 2021 13:07:13 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>> On Thu, 17 Jun 2021 22:04:34 +0530
>> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
>> 
>> > > 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?
>> 
>> Maybe or maybe not. At least with this approach (or the function graph
>> one), you will allocate enough for the environment involved. If there's
>> thousands of tasks, then yes, it will allocate more memory. But if you are
>> running thousands of tasks, you should have a lot of memory in the machine.
>> 
>> If you are only running a few tasks, it will be less than the current
>> approach.
> 
> Right, this depends on how many tasks you are running on your machine.
> Anyway, since you may not sure how much maxactive is enough, you will
> set maxactive high, then it can consume more than that. Of course you
> can optimize by trial and error. But that does not guarantee all cases,
> because the number of tasks can be increased while tracing. You might
> need to re-configure it by checking the nmissed count again.

Yes. If we go down this route, we should limit the per-task allocation 
to a more reasonable 4k -- powerpc uses 64k pages.

Thanks,
Naveen