netdev - Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance if its free list is empty

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Date:   Wed, 12 Apr 2017 08:42:26 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alban Crequy <alban.crequy@...il.com>,
        Alban Crequy <alban@...volk.io>,
        Alexei Starovoitov <ast@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Omar Sandoval <osandov@...com>, linux-doc@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        iago@...volk.io, michael@...volk.io,
        Dorau Lukasz <lukasz.dorau@...el.com>, systemtap@...rceware.org
Subject: Re: [RFC PATCH tip/master 2/3] kprobes: Allocate kretprobe instance
 if its free list is empty


* Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Thu, 30 Mar 2017 08:53:32 +0200
> Ingo Molnar <mingo@...nel.org> wrote:
> 
> > 
> > * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > 
> > > > So this is something I missed while the original code was merged, but the concept 
> > > > looks a bit weird: why do we do any "allocation" while a handler is executing?
> > > > 
> > > > That's fundamentally fragile. What's the maximum number of parallel 
> > > > 'kretprobe_instance' required per kretprobe - one per CPU?
> > > 
> > > It depends on the place where we put the probe. If the probed function will be
> > > blocked (yield to other tasks), then we need a same number of threads on
> > > the system which can invoke the function. So, ultimately, it is same
> > > as function_graph tracer, we need it for each thread.
> > 
> > So then put it into task_struct (assuming there's no kretprobe-inside-kretprobe 
> > nesting allowed).
> 
> No, that is possible to put several kretprobes on same thread, e.g.
> the func1() is called from func2(), user can put kretprobes for each
> function at same time.
> So the possible solution is to allocate new return-stack for each task_struct,
> and that is what the function-graph tracer did.
> 
> Anyway, I'm considering to integrate kretprobe_instance with the ret_stack.
> It will increase memory usage for kretprobes, but can provide safer way
> to allocate kretprobe_instance.

Ok, that sounds good to me.

Thanks,

	Ingo