Re: RCU related performance regression in 3.3

[Pascal Chapperon]

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
.
Do you want the rcu tracing log for this patch ?

Pascal

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