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

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Message-ID: <20200122071115.28e3c763@gandalf.local.home>
Date:   Wed, 22 Jan 2020 07:11:15 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.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 Wed, 22 Jan 2020 16:23:17 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> 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.
> 

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