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SubjectRe: [RFT PATCH 04/13] kprobes: Make optimizer delay to 1 second
On Wed, 22 Jan 2020 16:23:17 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Tue, 21 Jan 2020 19:29:05 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > On Thu, 16 Jan 2020 23:44:52 +0900
> > Masami Hiramatsu <mhiramat@kernel.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.
>

Perhaps we should have a mechanism that can detect new probes being
added, and just continue to delay the optimization, instead of having
some arbitrary delay.

-- Steve

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