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

[Petr Mladek <pmladek@suse.cz>]

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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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