On Thu, Mar 30, 2017 at 8:53 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * Masami Hiramatsu <mhira...@kernel.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). There's just no way in hell we should be calling any complex > kernel function from kernel probes!
Some kprobes are called from an interruption context. We have a kprobe on tcp_set_state() and this is sometimes called when the network card receives a packet. > I mean, think about it, a kretprobe can be installed in a lot of places, and > now > we want to call get_free_pages() from it?? This would add a massive amount of > fragility. > > Instrumentation must be _simple_, every patch that adds more complexity to the > most fundamental code path of it should raise a red flag ... > > So let's make this more robust, ok? > > Thanks, > > Ingo Thanks, Alban
