rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Petr Mladek]

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Hi,

I am doing some clean up in x86 ftrace code. I check the performance by
switching between different tracers and by enabling and disabling them.

The operation has started to be much slower after rebasing on the
kernel tip tree. Bisecting has shown that the difference was caused by
the commit c229828ca6bc62d6c654 (rcu: Throttle
rcu_try_advance_all_cbs() execution)

The following times are from Intel 2xCore i7-3770 CPU @ 3.40GHz when
calling the attached test script (time ./test-ftrace). It 100x switches
between "function" and "nop" tracer. It also enables and disables the
tracer each time.

Results with the commit c229828ca6bc62d6c654:

real    0m49.393s     0m49.632s     0m49.359s
user    0m0.004s      0m0.000s      0m0.004s
sys     0m0.996s      0m0.880s      0m0.892s


Results after reverting the commit c229828ca6bc62d6c654:

real    0m35.320s     0m35.687s     0m35.920s
user    0m0.004s      0m0.004s      0m0.000s
sys     0m1.140s      0m1.208s      0m1.152s


I might do some more debugging on Monday. I wonder if you have any
hints or ideas.

Best Regards,
Petr

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Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Paul E. McKenney]

On Fri, Nov 01, 2013 at 06:19:44PM +0100, Petr Mladek wrote:
> Hi,
> 
> I am doing some clean up in x86 ftrace code. I check the performance by
> switching between different tracers and by enabling and disabling them.
> 
> The operation has started to be much slower after rebasing on the
> kernel tip tree. Bisecting has shown that the difference was caused by
> the commit c229828ca6bc62d6c654 (rcu: Throttle
> rcu_try_advance_all_cbs() execution)
> 
> The following times are from Intel 2xCore i7-3770 CPU @ 3.40GHz when
> calling the attached test script (time ./test-ftrace). It 100x switches
> between "function" and "nop" tracer. It also enables and disables the
> tracer each time.
> 
> Results with the commit c229828ca6bc62d6c654:
> 
> real    0m49.393s     0m49.632s     0m49.359s
> user    0m0.004s      0m0.000s      0m0.004s
> sys     0m0.996s      0m0.880s      0m0.892s
> 
> 
> Results after reverting the commit c229828ca6bc62d6c654:
> 
> real    0m35.320s     0m35.687s     0m35.920s
> user    0m0.004s      0m0.004s      0m0.000s
> sys     0m1.140s      0m1.208s      0m1.152s
> 
> 
> I might do some more debugging on Monday. I wonder if you have any
> hints or ideas.

Hello, Petr,

This is a slowpath, and that commit did fix a real bug, so I am OK with
this modest slowdown.

That said, if you have a workload where this is a problem, please try
building with CONFIG_RCU_FAST_NO_HZ=n.  The fact that this commit had any
effect at all leads me to believe that you used CONFIG_RCU_FAST_NO_HZ=y.

							Thanx, Paul

Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Petr Mladek]

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Hello Paul,

Paul E. McKenney píše v Po 04. 11. 2013 v 09:02 -0800:
> On Fri, Nov 01, 2013 at 06:19:44PM +0100, Petr Mladek wrote:
> > Hi,
> > 
> > I am doing some clean up in x86 ftrace code. I check the performance by
> > switching between different tracers and by enabling and disabling them.
> > 
> > The operation has started to be much slower after rebasing on the
> > kernel tip tree. Bisecting has shown that the difference was caused by
> > the commit c229828ca6bc62d6c654 (rcu: Throttle
> > rcu_try_advance_all_cbs() execution)

> This is a slowpath, and that commit did fix a real bug, so I am OK with
> this modest slowdown.
> 
> That said, if you have a workload where this is a problem, please try
> building with CONFIG_RCU_FAST_NO_HZ=n.  The fact that this commit had any
> effect at all leads me to believe that you used CONFIG_RCU_FAST_NO_HZ=y.

Yes, I used CONFIG_RCU_FAST_NO_HZ=y.

I am not aware of any other workload with this problem. I tried few
benchmarks: dbench, unixbench, and aim. They did not show any
considerable difference with and without the commit. If you are
interested you might find more details in the attached logs.

Just for record, I also checked how the ftrace test was affected by the
commit under various system load. The speed difference was there if at
least one CPU was idle. But the test was slower on idle system even
without the patch. Hence this is not the only change that causes some
difference. See the attached "ftrace" file for more details.

I am still a bit curious why ftrace code is so special here and why it
does not affect the other benchmarks. Anyway, I agree that ftrace
change/start/stop operations are not time critical and the extra delay
might be worth fixing the other bug. I am fine with it :-)


Best Regards,
Petr

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test: ftrace
CPU: Intel(R) Core(TM)2 Duo CPU     E8400  @ 3.00GHz
RAM: 4GB

Started using the command:

	time ./test-ftrace

Where test-ftrace is:

--- cut ---
#!/bin/bash

echo "Testing ftrace - begin"
cd /sys/kernel/debug/tracing

count=0
while test $count -lt 100 ; do
    for tracer in function nop ; do
	echo $tracer >current_tracer
	echo 1 >tracing_on
	echo 0 >tracing_on
    done
    count=$(($count + 1))
done

cd -
echo "Testing ftrace - end"
--- cut ---

and the load was generated using the script make-load:

--- cut ---
#!/bin/bash

a=1
while true ; do
    a=$(($a+1))
done
--- cut ---

Now, results with no load on the system; with the commit c229828ca6bc62d
(rcu: Throttle rcu_try_advance_all_cbs() execution)

real    0m49.393s    0m49.632s    0m49.359
user    0m0.004s     0m0.000s	  0m0.004s
sys     0m0.996s     0m0.880s	  0m0.892s

and with reverted c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
execution)

real    0m35.320s    0m35.687s    0m35.920s
user    0m0.004s     0m0.004s	  0m0.000s
sys     0m1.140s     0m1.208s	  0m1.152s

---

Results with 100% load on one CPU and no load on 2nd CPU (started the script
make-load once) with c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
execution)

real    0m44.964s    0m45.083s    0m45.171s
user    0m0.000s     0m0.000s	  0m0.004s
sys     0m1.580s     0m1.612s	  0m1.576s

and with reverted c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
execution)

real    0m30.816s    0m31.326s    0m31.191s
user    0m0.004s     0m0.004s	  0m0.008s
sys     0m1.240s     0m1.248s	  0m1.232s

---

Results with 100% load on both CPUs (started script make-load twice)
with c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs() execution)

real    0m20.294s    0m20.538s    0m20.470s
user    0m0.004s     0m0.000s	  0m0.000s
sys     0m1.284s     0m1.388s	  0m1.612s

and with reverted c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
execution)

real    0m20.305s    0m20.056s    0m20.259
user    0m0.000s     0m0.000s	  0m0.000s
sys     0m1.416s     0m1.452s	  0m1.560s

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Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Paul E. McKenney]

On Tue, Nov 05, 2013 at 04:43:06PM +0100, Petr Mladek wrote:
> Hello Paul,
> 
> Paul E. McKenney píše v Po 04. 11. 2013 v 09:02 -0800:
> > On Fri, Nov 01, 2013 at 06:19:44PM +0100, Petr Mladek wrote:
> > > Hi,
> > > 
> > > I am doing some clean up in x86 ftrace code. I check the performance by
> > > switching between different tracers and by enabling and disabling them.
> > > 
> > > The operation has started to be much slower after rebasing on the
> > > kernel tip tree. Bisecting has shown that the difference was caused by
> > > the commit c229828ca6bc62d6c654 (rcu: Throttle
> > > rcu_try_advance_all_cbs() execution)
> 
> > This is a slowpath, and that commit did fix a real bug, so I am OK with
> > this modest slowdown.
> > 
> > That said, if you have a workload where this is a problem, please try
> > building with CONFIG_RCU_FAST_NO_HZ=n.  The fact that this commit had any
> > effect at all leads me to believe that you used CONFIG_RCU_FAST_NO_HZ=y.
> 
> Yes, I used CONFIG_RCU_FAST_NO_HZ=y.
> 
> I am not aware of any other workload with this problem. I tried few
> benchmarks: dbench, unixbench, and aim. They did not show any
> considerable difference with and without the commit. If you are
> interested you might find more details in the attached logs.
> 
> Just for record, I also checked how the ftrace test was affected by the
> commit under various system load. The speed difference was there if at
> least one CPU was idle. But the test was slower on idle system even
> without the patch. Hence this is not the only change that causes some
> difference. See the attached "ftrace" file for more details.

Ah, the joys of energy efficiency!  ;-)

If the system is entirely non-idle, each CPU will respond to RCU in a
timely fashion.  However, RCU cannot be permitted to interrupt or in any
way disturb an idle CPU, most especially on a battery-powered system.
To do so would completely destroy energy efficiency and battery lifetime.
Instead, idle CPUs leave evidence of their idleness in per-CPU variables,
and RCU must check these variables for any CPUs that do not respond in
a timely fashion.

Of course, it would not do for RCU to be in a busy-waiting loop
continuously checking these variables.  Instead, RCU checks them
after a three-jiffy delay, which can be tuned if desired using the
rcutree.jiffies_till_first_fqs kernel boot parameter.  You can even set it
to zero if you like, which should speed up your ftrace test significantly
on idle systems.  You might also tune the rcutree.jiffies_till_next_fqs
kernel boot parameter, which governs the time delay until the second
and subsequent checks for a given RCU grace period.

The reason that the change mostly affected idle systems is that
CONFIG_RCU_FAST_NO_HZ=y (and thus the change to its code) affects
the transitions to and from idle.  If your system is never idle,
then CONFIG_RCU_FAST_NO_HZ=y will have no effect whatsoever.

> I am still a bit curious why ftrace code is so special here and why it
> does not affect the other benchmarks. Anyway, I agree that ftrace
> change/start/stop operations are not time critical and the extra delay
> might be worth fixing the other bug. I am fine with it :-)

The ftrace code makes heavy use of RCU updates when enabling and disabling
things, which makes its performance quite sensitive to grace-period
duration, and (as you saw) grace-period duration is increased somewhat
by CONFIG_RCU_FAST_NO_HZ=y.  This trade of increased energy efficiency
for longer grace-period duration is the right trade for most systems.

Other systems can use CONFIG_RCU_FAST_NO_HZ=n and/or tune the
rcutree.jiffies_till_first_fqs and rcutree.jiffies_till_next_fqs
kernel boot parameters.

Hey, you asked!  ;-)

							Thanx, Paul

> Best Regards,
> Petr



> test: ftrace
> CPU: Intel(R) Core(TM)2 Duo CPU     E8400  @ 3.00GHz
> RAM: 4GB
> 
> Started using the command:
> 
> 	time ./test-ftrace
> 
> Where test-ftrace is:
> 
> --- cut ---
> #!/bin/bash
> 
> echo "Testing ftrace - begin"
> cd /sys/kernel/debug/tracing
> 
> count=0
> while test $count -lt 100 ; do
>     for tracer in function nop ; do
> 	echo $tracer >current_tracer
> 	echo 1 >tracing_on
> 	echo 0 >tracing_on
>     done
>     count=$(($count + 1))
> done
> 
> cd -
> echo "Testing ftrace - end"
> --- cut ---
> 
> and the load was generated using the script make-load:
> 
> --- cut ---
> #!/bin/bash
> 
> a=1
> while true ; do
>     a=$(($a+1))
> done
> --- cut ---
> 
> Now, results with no load on the system; with the commit c229828ca6bc62d
> (rcu: Throttle rcu_try_advance_all_cbs() execution)
> 
> real    0m49.393s    0m49.632s    0m49.359
> user    0m0.004s     0m0.000s	  0m0.004s
> sys     0m0.996s     0m0.880s	  0m0.892s
> 
> and with reverted c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
> execution)
> 
> real    0m35.320s    0m35.687s    0m35.920s
> user    0m0.004s     0m0.004s	  0m0.000s
> sys     0m1.140s     0m1.208s	  0m1.152s
> 
> ---
> 
> Results with 100% load on one CPU and no load on 2nd CPU (started the script
> make-load once) with c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
> execution)
> 
> real    0m44.964s    0m45.083s    0m45.171s
> user    0m0.000s     0m0.000s	  0m0.004s
> sys     0m1.580s     0m1.612s	  0m1.576s
> 
> and with reverted c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
> execution)
> 
> real    0m30.816s    0m31.326s    0m31.191s
> user    0m0.004s     0m0.004s	  0m0.008s
> sys     0m1.240s     0m1.248s	  0m1.232s
> 
> ---
> 
> Results with 100% load on both CPUs (started script make-load twice)
> with c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs() execution)
> 
> real    0m20.294s    0m20.538s    0m20.470s
> user    0m0.004s     0m0.000s	  0m0.000s
> sys     0m1.284s     0m1.388s	  0m1.612s
> 
> and with reverted c229828ca6bc62d (rcu: Throttle rcu_try_advance_all_cbs()
> execution)
> 
> real    0m20.305s    0m20.056s    0m20.259
> user    0m0.000s     0m0.000s	  0m0.000s
> sys     0m1.416s     0m1.452s	  0m1.560s

Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Fengguang Wu]

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Hi Petr,

Sorry for picking up this old thread, but I noticed your attached
ftrace test script and would like to ask for your permission to
include it in

        https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git

which is GPLv2. If you kindly agree, I'll run it actively for testing
the upstream linux kernels.

Thanks,
Fengguang

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Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Petr Mládek]

On Fri 2014-08-22 11:35:29, Fengguang Wu wrote:
> Hi Petr,
> 
> Sorry for picking up this old thread, but I noticed your attached
> ftrace test script and would like to ask for your permission to
> include it in
> 
>         https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git
> 
> which is GPLv2. If you kindly agree, I'll run it actively for testing
> the upstream linux kernels.

Sure. Feel free to use the test script under GPLv2 license.

Best Regards,
Petr

Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Fengguang Wu]

On Fri, Aug 22, 2014 at 10:32:57AM +0200, Petr Mládek wrote:
> On Fri 2014-08-22 11:35:29, Fengguang Wu wrote:
> > Hi Petr,
> > 
> > Sorry for picking up this old thread, but I noticed your attached
> > ftrace test script and would like to ask for your permission to
> > include it in
> > 
> >         https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git
> > 
> > which is GPLv2. If you kindly agree, I'll run it actively for testing
> > the upstream linux kernels.
> 
> Sure. Feel free to use the test script under GPLv2 license.

Great, thank you very much!

Fengguang

Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Steven Rostedt]

On Fri, 22 Aug 2014 19:43:02 +0800
Fengguang Wu <fengguang.wu@intel.com> wrote:

> On Fri, Aug 22, 2014 at 10:32:57AM +0200, Petr Mládek wrote:
> > On Fri 2014-08-22 11:35:29, Fengguang Wu wrote:
> > > Hi Petr,
> > > 
> > > Sorry for picking up this old thread, but I noticed your attached
> > > ftrace test script and would like to ask for your permission to
> > > include it in
> > > 
> > >         https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git
> > > 
> > > which is GPLv2. If you kindly agree, I'll run it actively for testing
> > > the upstream linux kernels.
> > 
> > Sure. Feel free to use the test script under GPLv2 license.
> 
> Great, thank you very much!
> 
> Fengguang

Sorry for the very late reply, I've been on vacation and then had some
medical issues.

Note, I'm working with Masami to include lots of ftrace tests that will
supersede this script. The tests will be included in
tools/testing/selftests. Look for the subject "ftracetest".

-- Steve

Re: rcu: Throttle rcu_try_advance_all_cbs() execution causes visible slowdown in ftrace switching

[Fengguang Wu]

HI Steven,

On Wed, Sep 17, 2014 at 10:42:51AM -0400, Steven Rostedt wrote:
> On Fri, 22 Aug 2014 19:43:02 +0800
> Fengguang Wu <fengguang.wu@intel.com> wrote:
> 
> > On Fri, Aug 22, 2014 at 10:32:57AM +0200, Petr Mládek wrote:
> > > On Fri 2014-08-22 11:35:29, Fengguang Wu wrote:
> > > > Hi Petr,
> > > > 
> > > > Sorry for picking up this old thread, but I noticed your attached
> > > > ftrace test script and would like to ask for your permission to
> > > > include it in
> > > > 
> > > >         https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git
> > > > 
> > > > which is GPLv2. If you kindly agree, I'll run it actively for testing
> > > > the upstream linux kernels.
> > > 
> > > Sure. Feel free to use the test script under GPLv2 license.
> > 
> > Great, thank you very much!
> > 
> > Fengguang
> 
> Sorry for the very late reply, I've been on vacation and then had some
> medical issues.
> 
> Note, I'm working with Masami to include lots of ftrace tests that will
> supersede this script. The tests will be included in
> tools/testing/selftests. Look for the subject "ftracetest".

Yes I just see the patches from Masami. We'll add them to the LKP
test suites. Thank you very much for letting me know!

Regards,
Fengguang