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Message-Id: <20200123103334.6e1821625643d007297ecf94@kernel.org>
Date: Thu, 23 Jan 2020 10:33:34 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: paulmck@...nel.org
Cc: Steven Rostedt <rostedt@...dmis.org>,
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>, 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 08:54:32 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:
> On Wed, Jan 22, 2020 at 10:12:40PM +0900, Masami Hiramatsu wrote:
> > On Wed, 22 Jan 2020 07:11:15 -0500
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> >
> > > 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.
> >
> > Yes, that is what [03/13] does :)
> > Anyway, it seems that the RCU-synchronization takes more than 5 jiffies.
> > And in that case, [03/13] still doesn't work. That's why I added this patch
> > after that.
>
> If the RCU synchronization is synchronize_rcu_tasks(), then yes, it
> will often take way more than 5 jiffies. If it is synchronize_rcu(),
> 5 jiffies would not be unusual, especially on larger systems.
> But in the case of synchronize_rcu(), one option is to instead use
> synchronize_rcu_expedited(). It is not clear that this last is really
> justified in this case, but figured it might be worth mentioning.
It is synchronize_rcu(), but it seems sometimes it is called several
times on one probe disabling.
Anyway, without this update, I added a printk to count optimizer
queue-length and found that all optimizer call with a single kprobe
on the quenes. I think this depends on the machine, but as far as
I tested on 8-threads qemu x86, shows this result...
Probes: 256 kprobe_events
Enable events
real 0m 0.02s
user 0m 0.00s
sys 0m 0.02s
[ 17.730548] Queue-update: 180, skipped, Total Queued: 180
[ 17.739445] Queue-update: 1, go, Total Queued: 180
Disable events
[ 19.744634] Queue-update: 1, go, Total Queued: 1
[ 19.770743] Queue-update: 1, go, Total Queued: 1
[ 19.886625] Queue-update: 1, go, Total Queued: 1
[ 20.006631] Queue-update: 1, go, Total Queued: 1
[ 20.126628] Queue-update: 1, go, Total Queued: 1
[ 20.246620] Queue-update: 1, go, Total Queued: 1
[ 20.374665] Queue-update: 1, go, Total Queued: 1
[ 20.486617] Queue-update: 1, go, Total Queued: 1
[ 20.606608] Queue-update: 1, go, Total Queued: 1
[ 20.726596] Queue-update: 1, go, Total Queued: 1
[ 20.846608] Queue-update: 1, go, Total Queued: 1
[ 20.966723] Queue-update: 1, go, Total Queued: 1
[ 21.086611] Queue-update: 1, go, Total Queued: 1
[ 21.206605] Queue-update: 1, go, Total Queued: 1
[ 21.326603] Queue-update: 1, go, Total Queued: 1
[ 21.462609] Queue-update: 1, go, Total Queued: 1
[ 21.566755] Queue-update: 1, go, Total Queued: 1
[ 21.686606] Queue-update: 1, go, Total Queued: 1
[ 21.806611] Queue-update: 1, go, Total Queued: 1
[ 21.926609] Queue-update: 1, go, Total Queued: 1
[ 22.046621] Queue-update: 1, go, Total Queued: 1
[ 22.166729] Queue-update: 1, go, Total Queued: 1
[ 22.302609] Queue-update: 1, go, Total Queued: 1
[ 22.510627] Queue-update: 1, go, Total Queued: 1
[ 22.536638] Queue-update: 1, go, Total Queued: 1
[ 22.654618] Queue-update: 1, go, Total Queued: 1
[ 22.774643] Queue-update: 1, go, Total Queued: 1
[ 22.902609] Queue-update: 1, go, Total Queued: 1
[ 23.022608] Queue-update: 1, go, Total Queued: 1
[ 23.158606] Queue-update: 1, go, Total Queued: 1
[ 23.254618] Queue-update: 1, go, Total Queued: 1
[ 23.374647] Queue-update: 1, go, Total Queued: 1
[ 23.494605] Queue-update: 1, go, Total Queued: 1
[ 23.614610] Queue-update: 1, go, Total Queued: 1
[ 23.734606] Queue-update: 1, go, Total Queued: 1
[ 23.854609] Queue-update: 1, go, Total Queued: 1
[ 23.974615] Queue-update: 1, go, Total Queued: 1
[ 24.094609] Queue-update: 1, go, Total Queued: 1
[ 24.230607] Queue-update: 1, go, Total Queued: 1
[ 24.342625] Queue-update: 1, go, Total Queued: 1
[ 24.475478] Queue-update: 1, go, Total Queued: 1
[ 24.574554] Queue-update: 1, go, Total Queued: 1
[ 24.694605] Queue-update: 1, go, Total Queued: 1
[ 24.814585] Queue-update: 1, go, Total Queued: 1
[ 24.934591] Queue-update: 1, go, Total Queued: 1
[ 25.054614] Queue-update: 1, go, Total Queued: 1
[ 25.174628] Queue-update: 1, go, Total Queued: 1
[ 25.294637] Queue-update: 1, go, Total Queued: 1
[ 25.414620] Queue-update: 1, go, Total Queued: 1
[ 25.534553] Queue-update: 1, go, Total Queued: 1
[ 25.654610] Queue-update: 1, go, Total Queued: 1
[ 25.774627] Queue-update: 1, go, Total Queued: 1
[ 25.894609] Queue-update: 1, go, Total Queued: 1
[ 26.030548] Queue-update: 1, go, Total Queued: 1
[ 26.134626] Queue-update: 1, go, Total Queued: 1
[ 26.254620] Queue-update: 1, go, Total Queued: 1
[ 26.373603] Queue-update: 1, go, Total Queued: 1
[ 26.502606] Queue-update: 1, go, Total Queued: 1
[ 26.614607] Queue-update: 1, go, Total Queued: 1
[ 26.734610] Queue-update: 1, go, Total Queued: 1
[ 26.854620] Queue-update: 1, go, Total Queued: 1
[ 26.974660] Queue-update: 1, go, Total Queued: 1
[ 27.094620] Queue-update: 1, go, Total Queued: 1
[ 27.214596] Queue-update: 1, go, Total Queued: 1
[ 27.334640] Queue-update: 1, go, Total Queued: 1
[ 27.494606] Queue-update: 1, go, Total Queued: 1
[ 27.574703] Queue-update: 1, go, Total Queued: 1
[ 27.694609] Queue-update: 1, go, Total Queued: 1
[ 27.814607] Queue-update: 1, go, Total Queued: 1
[ 27.934631] Queue-update: 1, go, Total Queued: 1
[ 28.054606] Queue-update: 1, go, Total Queued: 1
[ 28.182685] Queue-update: 1, go, Total Queued: 1
[ 28.318568] Queue-update: 1, go, Total Queued: 1
[ 28.422610] Queue-update: 1, go, Total Queued: 1
[ 28.534621] Queue-update: 1, go, Total Queued: 1
[ 28.654618] Queue-update: 1, go, Total Queued: 1
[ 28.774622] Queue-update: 1, go, Total Queued: 1
[ 28.894609] Queue-update: 1, go, Total Queued: 1
[ 29.022609] Queue-update: 1, go, Total Queued: 1
[ 29.150615] Queue-update: 1, go, Total Queued: 1
[ 29.262610] Queue-update: 1, go, Total Queued: 1
[ 29.374621] Queue-update: 1, go, Total Queued: 1
[ 29.494606] Queue-update: 1, go, Total Queued: 1
[ 29.614616] Queue-update: 1, go, Total Queued: 1
[ 29.734607] Queue-update: 1, go, Total Queued: 1
[ 29.854601] Queue-update: 1, go, Total Queued: 1
[ 29.974610] Queue-update: 1, go, Total Queued: 1
[ 30.094625] Queue-update: 1, go, Total Queued: 1
[ 30.214606] Queue-update: 1, go, Total Queued: 1
[ 30.334602] Queue-update: 1, go, Total Queued: 1
[ 30.454634] Queue-update: 1, go, Total Queued: 1
[ 30.574606] Queue-update: 1, go, Total Queued: 1
[ 30.694589] Queue-update: 1, go, Total Queued: 1
[ 30.814613] Queue-update: 1, go, Total Queued: 1
[ 30.934602] Queue-update: 1, go, Total Queued: 1
[ 31.054605] Queue-update: 1, go, Total Queued: 1
[ 31.182596] Queue-update: 1, go, Total Queued: 1
[ 31.318621] Queue-update: 1, go, Total Queued: 1
[ 31.414615] Queue-update: 1, go, Total Queued: 1
[ 31.534610] Queue-update: 1, go, Total Queued: 1
[ 31.670608] Queue-update: 1, go, Total Queued: 1
[ 31.774626] Queue-update: 1, go, Total Queued: 1
[ 31.894607] Queue-update: 1, go, Total Queued: 1
[ 32.014609] Queue-update: 1, go, Total Queued: 1
[ 32.134607] Queue-update: 1, go, Total Queued: 1
[ 32.254611] Queue-update: 1, go, Total Queued: 1
[ 32.374608] Queue-update: 1, go, Total Queued: 1
[ 32.494619] Queue-update: 1, go, Total Queued: 1
[ 32.614607] Queue-update: 1, go, Total Queued: 1
[ 32.734612] Queue-update: 1, go, Total Queued: 1
[ 32.862616] Queue-update: 1, go, Total Queued: 1
[ 32.974620] Queue-update: 1, go, Total Queued: 1
[ 33.110609] Queue-update: 1, go, Total Queued: 1
[ 33.214609] Queue-update: 1, go, Total Queued: 1
[ 33.342611] Queue-update: 1, go, Total Queued: 1
[ 33.454607] Queue-update: 1, go, Total Queued: 1
[ 33.574611] Queue-update: 1, go, Total Queued: 1
[ 33.694619] Queue-update: 1, go, Total Queued: 1
[ 33.814607] Queue-update: 1, go, Total Queued: 1
[ 33.950614] Queue-update: 1, go, Total Queued: 1
[ 34.062609] Queue-update: 1, go, Total Queued: 1
[ 34.174609] Queue-update: 1, go, Total Queued: 1
[ 34.294619] Queue-update: 1, go, Total Queued: 1
[ 34.430533] Queue-update: 1, go, Total Queued: 1
[ 34.534594] Queue-update: 1, go, Total Queued: 1
[ 34.654605] Queue-update: 1, go, Total Queued: 1
[ 34.790596] Queue-update: 1, go, Total Queued: 1
[ 34.902611] Queue-update: 1, go, Total Queued: 1
[ 35.014630] Queue-update: 1, go, Total Queued: 1
[ 35.134634] Queue-update: 1, go, Total Queued: 1
[ 35.262608] Queue-update: 1, go, Total Queued: 1
[ 35.374634] Queue-update: 1, go, Total Queued: 1
[ 35.494617] Queue-update: 1, go, Total Queued: 1
[ 35.622608] Queue-update: 1, go, Total Queued: 1
[ 35.742610] Queue-update: 1, go, Total Queued: 1
[ 35.854572] Queue-update: 1, go, Total Queued: 1
[ 35.982596] Queue-update: 1, go, Total Queued: 1
[ 36.094603] Queue-update: 1, go, Total Queued: 1
[ 36.222612] Queue-update: 1, go, Total Queued: 1
[ 36.334610] Queue-update: 1, go, Total Queued: 1
[ 36.454619] Queue-update: 1, go, Total Queued: 1
[ 36.574619] Queue-update: 1, go, Total Queued: 1
[ 36.694643] Queue-update: 1, go, Total Queued: 1
[ 36.814614] Queue-update: 1, go, Total Queued: 1
[ 36.934610] Queue-update: 1, go, Total Queued: 1
[ 37.062609] Queue-update: 1, go, Total Queued: 1
[ 37.198611] Queue-update: 1, go, Total Queued: 1
[ 37.294618] Queue-update: 1, go, Total Queued: 1
[ 37.414618] Queue-update: 1, go, Total Queued: 1
[ 37.534595] Queue-update: 1, go, Total Queued: 1
[ 37.662594] Queue-update: 1, go, Total Queued: 1
[ 37.774610] Queue-update: 1, go, Total Queued: 1
[ 37.894618] Queue-update: 1, go, Total Queued: 1
[ 38.014619] Queue-update: 1, go, Total Queued: 1
[ 38.142612] Queue-update: 1, go, Total Queued: 1
[ 38.254609] Queue-update: 1, go, Total Queued: 1
[ 38.374619] Queue-update: 1, go, Total Queued: 1
[ 38.502481] Queue-update: 1, go, Total Queued: 1
[ 38.614596] Queue-update: 1, go, Total Queued: 1
[ 38.734597] Queue-update: 1, go, Total Queued: 1
[ 38.854606] Queue-update: 1, go, Total Queued: 1
[ 38.974620] Queue-update: 1, go, Total Queued: 1
[ 39.094617] Queue-update: 1, go, Total Queued: 1
[ 39.222597] Queue-update: 1, go, Total Queued: 1
[ 39.334610] Queue-update: 1, go, Total Queued: 1
[ 39.454609] Queue-update: 1, go, Total Queued: 1
[ 39.574633] Queue-update: 1, go, Total Queued: 1
[ 39.694611] Queue-update: 1, go, Total Queued: 1
[ 39.814608] Queue-update: 1, go, Total Queued: 1
[ 39.934610] Queue-update: 1, go, Total Queued: 1
[ 40.054621] Queue-update: 1, go, Total Queued: 1
[ 40.174611] Queue-update: 1, go, Total Queued: 1
[ 40.297471] Queue-update: 1, go, Total Queued: 1
[ 40.414504] Queue-update: 1, go, Total Queued: 1
[ 40.534601] Queue-update: 1, go, Total Queued: 1
[ 40.654611] Queue-update: 1, go, Total Queued: 1
[ 40.774609] Queue-update: 1, go, Total Queued: 1
[ 40.894628] Queue-update: 1, go, Total Queued: 1
[ 41.014608] Queue-update: 1, go, Total Queued: 1
[ 41.135594] Queue-update: 1, go, Total Queued: 1
real 0m 21.40s
user 0m 0.00s
sys 0m 0.04s
Remove events
real 0m 2.14s
user 0m 0.00s
sys 0m 0.04s
Thank you,
--
Masami Hiramatsu <mhiramat@...nel.org>
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