Re: RCU related performance regression in 3.3

[Pascal Chapperon]

Le 18/05/2012 14:14, Paul E. McKenney a Ãcrit :
> On Fri, May 18, 2012 at 01:01:41PM +0200, Pascal Chapperon wrote:
> > Le 15/05/2012 00:32, Paul E. McKenney a Ãcrit :
> > > On Fri, May 04, 2012 at 04:14:42PM -0700, Paul E. McKenney wrote:
> > > > On Fri, May 04, 2012 at 11:41:13PM +0200, Pascal Chapperon wrote:
> > > > > Le 04/05/2012 17:04, Paul E. McKenney a Ãcrit :
> > > > > > On Fri, May 04, 2012 at 04:42:54PM +0200, Pascal Chapperon wrote:
> > > > > > > Le 01/05/2012 17:45, Paul E. McKenney a Ãcrit :
> > > > > > >
> > > > > > > > Here is my RCU_FAST_NO_HZ patch stack on top of v3.4-rc4.
> > > > > > > >
> > > > > > > > Or you can pull branch fnh.2012.05.01a from:
> > > > > > > >
> > > > > > > > 	git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git
> > > > > > > >
> > > > > > > > 							Thanx, Paul
> > > > > > > I applied your global patch on top of v3.4-rc4. But the slowdown is
> > > > > > > worse than before : boot sequence took 80s instead 20-30s (12s for
> > > > > > > initramfs instead of 2s).
> > > > > > >
> > > > > > > I'll send you rcu tracing log in a second mail.
> > > > > >
> > > > > > Hmmm...  Well, I guess I am glad that I finally did something that
> > > > > > had an effect, but I sure wish that the effect had been in the other
> > > > > > direction!
> > > > > >
> > > > > > Just to make sure I understand: the difference between the 20-30s and
> > > > > > the 80s is exactly the patch I sent you?
> > > > > >
> > > > > > 							Thanx, Paul
> > > > > Yes. Exactly same kernel config as in previous results, I applied
> > > > > your patch against v3.4-rc4, and sorry, the result is exactly what I
> > > > > said;
> > > > > I saw that your global patch was quite huge, and addresses things which
> > > > > are not directly related with the initial patch (commit
> > > > > 7cb92499000e3c86dae653077b1465458a039ef6); maybe a side effect?
> > > > >
> > > > > However, I'm ready to try this patch on my smaller laptop which
> > > > > supports well CONFIG_FAST_NO_HZ=y and systemd, if you think it can
> > > > > help ?
> > > > >
> > > > > Another thought: this issue as nothing to do with i7 Hyper-threading
> > > > > capacities ? (as I test core2duo, Pentium ulv in same conditions and I
> > > > > don't encountered any slowdown ?)
> > > >
> > > > Well, one possibility is that your setup starts the jiffies counter
> > > > at some interesting value.  The attached patch (also against v3.4-rc4)
> > > > applies a bit more paranoia to the initialization to handle this
> > > > and other possibilities.
> > >
> > > This patchset fixes the problem where RCU_FAST_NO_HZ's timers were
> > > being ignored due to the dyntick-idle code having already calculated
> > > the CPU's wakeup time (which I sent earlier, mistakenly offlist), but
> > > also fixes a botched check in my workaround.
> > >
> > > Could you please try it out?  This patch is against 3.4-rc4.
> > >
> > > 							Thanx, Paul
> > Hi Paul,
> >
> > <  +     if (!rcu_cpu_has_nonlazy_callbacks(cpu))
> > ---
> > > +     if (rcu_cpu_has_nonlazy_callbacks(cpu))
> >
> > I was a little disappointed by the previous patch (boot sequence still
> > took 72 s.), but this one makes a huge difference ;-)
> > Slowdown during boot or shutdown with CONFIG_RCU_FAST_NO_HZ has
> > disappeared (~ 10 attempts) :
> > # systemd-analyze
> > Startup finished in 1990ms (kernel) + 1174ms (initramfs) + 3121ms
> > (userspace) = 6285ms
> > .
>
> Very good!  And thank you very much for all your testing efforts and
> for bearing with me through this!
>
> Does this mean that I can add your Tested-by?

Yes: the results are good and stable, at least for my hardware.
I tried with both a standard fedora 16 kernel configuration and a custom
one (hardware optimized, preempted, etc...) and this on over more 20
attempts.
With or without FAST_NO_HZ makes no difference now.

> > Do you want the rcu tracing log for this patch ?
>
> Could you please?  Just in case there is some other surprise that
> I should know about that might not be visible.  ;-)
>
> 							Thanx, Paul
I'll send you the logs in a second mail (offlist).

Pascal


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