linux-kernel - Re: [RFT PATCH 04/13] kprobes: Make optimizer delay to 1 second

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Date:   Wed, 22 Jan 2020 16:23:17 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Brendan Gregg <brendan.d.gregg@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        "David S . Miller" <davem@...emloft.net>, paulmck@...nel.org,
        joel@...lfernandes.org,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>
Subject: Re: [RFT PATCH 04/13] kprobes: Make optimizer delay to 1 second

On Tue, 21 Jan 2020 19:29:05 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Thu, 16 Jan 2020 23:44:52 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > Since the 5 jiffies delay for the optimizer is too
> > short to wait for other probes, make it longer,
> > like 1 second.
> 
> Hi Masami,
> 
> Can you explain more *why* 5 jiffies is too short.

Yes, I had introduced this 5 jiffies delay for multiple probe registration
and unregistration like systemtap, which will use array-based interface to
register/unregister. In that case, 5 jiffies will be enough for the delay
to wait for other kprobe registration/unregsitration.

However, since perf and ftrace register/unregister probes one-by-one with
RCU synchronization interval, the optimizer will be started before
finishing to register/unregister all probes.
And the optimizer locks kprobe_mutex a while -- RCU-tasks synchronization.
Since the kprobe_mutex is also involved in disabling kprobes, this also
stops probe-event disabling.

Maybe 5 jiffies is enough for adding/removing a few probe events, but
not enough for dozens of probe events.

Thank you,

> 
> Thanks!
> 
> -- Steve
> 
> > 
> > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > ---
> >  kernel/kprobes.c |    3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index 0dacdcecc90f..9c6e230852ad 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -469,7 +469,8 @@ static int kprobe_optimizer_queue_update;
> >  
> >  static void kprobe_optimizer(struct work_struct *work);
> >  static DECLARE_DELAYED_WORK(optimizing_work, kprobe_optimizer);
> > -#define OPTIMIZE_DELAY 5
> > +/* Wait 1 second for starting optimization */
> > +#define OPTIMIZE_DELAY HZ
> >  
> >  /*
> >   * Optimize (replace a breakpoint with a jump) kprobes listed on
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>