Messages in this thread Patch in this message | | | Date | Tue, 10 Apr 2012 09:07:06 -0700 | From | "Paul E. McKenney" <> | Subject | Re: RCU related performance regression in 3.3 |
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On Fri, Apr 06, 2012 at 11:18:03AM +0200, Pascal Chapperon wrote: > > Message du 05/04/12 16:40 > > De : "Paul E. McKenney" > > A : "Pascal CHAPPERON" > > Copie à : "Josh Boyer" , linux-kernel@vger.kernel.org, kernel-team@fedoraproject.org > > Objet : Re: RCU related performance regression in 3.3 > > > > On Thu, Apr 05, 2012 at 04:15:33PM +0200, Pascal CHAPPERON wrote: > > > Hello, > > > > > > I didn't notice any significant slowdown while the system is up and running. > A full kernel compilation (make -j 16) takes 14mn with both 3.2.10 and 3.3.0.
OK, good.
> > OK, so the natural approach is to disable CONFIG_RCU_FAST_NO_HZ at > > boot time. Unfortunately, you appear to need it to remain disabled > > through at least filesystem mounting, which if I understand correctly > > happens long after system_state gets set to SYSTEM_RUNNING. > > > In fact, I need it to remain disable until all the systemd units are completed. > Some units, such as NetworkManager can take longer time to complete with > RCU_FAST_NO_HZ enabled. > And i need it to be disabled at shutdown, as umounting cgroups, sysfs, etc. > plus old-root mounting can take one plain second for each umounting.
OK...
> > If RCU has some way to find out when init is complete, I can easily > > make it so that CONFIG_RCU_FAST_NO_HZ optimizes for speed during boot > > and energy efficiency during runtime. > > > I said that I didn't noticed significant slowdown during runtime, but my > laptop usage is basic. Some specific tasks similar to systemd may > perhaps be impacted by this feature. > I can test a task/program that could stress RCU_FAST_NO_HZ if any ?
One thing to try first -- could you please check boot/shutdown slowdown with the patch below?
But yes, there are things like modifying netfilter rules and updating security configuration that might be affected.
> > One thing I could easily do would be to provide a sysfs parameter or > > some such that allows the boot process to enable energy-efficiency > > mode at runtime. I would much prefer to make this automatic, though. > > > So the feature is disabled until you trigger a sysfs parameter, and can be > disabled before shutdown ? It would be fair, at least for hardware like my > own.
That is the thought, though again I would really really prefer that this be automated.
> > Other thoughts? > > > Do you think that the culprit is a buggy hardware in my laptop, or the > number of cpu/threads ?
Maybe just more filesystems to mount?
> > Thanx, Paul > > > > Resent in plain text because rejected. Sorry, i forgot the rules.
No problem! Please see below for the patch.
Thanx, Paul
------------------------------------------------------------------------
rcu: Decrease RCU_FAST_NO_HZ timer interval
Use of RCU_FAST_NO_HZ reportedly significantly slowed boot time in configurations involving lots of filesystem mounts. This commit attempts to work around this by decreasing the RCU_FAST_NO_HZ timer interval from 6 jiffies down to 3 jiffies.
Reported-by: Pascal Chapperon <pascal.chapperon@wanadoo.fr> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h index c023464..980a4bb 100644 --- a/kernel/rcutree_plugin.h +++ b/kernel/rcutree_plugin.h @@ -1975,7 +1975,7 @@ static void rcu_prepare_for_idle(int cpu) */ #define RCU_IDLE_FLUSHES 5 /* Number of dyntick-idle tries. */ #define RCU_IDLE_OPT_FLUSHES 3 /* Optional dyntick-idle tries. */ -#define RCU_IDLE_GP_DELAY 6 /* Roughly one grace period. */ +#define RCU_IDLE_GP_DELAY 3 /* Roughly one grace period. */ #define RCU_IDLE_LAZY_GP_DELAY (6 * HZ) /* Roughly six seconds. */ static DEFINE_PER_CPU(int, rcu_dyntick_drain); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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