Messages in this thread | | | Date | Fri, 18 May 2012 05:14:01 -0700 | From | "Paul E. McKenney" <> | Subject | Re: RCU related performance regression in 3.3 |
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On Fri, May 18, 2012 at 01:01:41PM +0200, Pascal Chapperon wrote: > Le 15/05/2012 00:32, Paul E. McKenney a écrit : > >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 écrit : > >>>>On Fri, May 04, 2012 at 04:42:54PM +0200, Pascal Chapperon wrote: > >>>>>Le 01/05/2012 17:45, Paul E. McKenney a écrit : > >>>>> > >>>>>>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?
> 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
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