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Message-ID: <20090322051822.GG7148@linux.vnet.ibm.com> Date: Sat, 21 Mar 2009 22:18:22 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Arjan van de Ven <arjan@...radead.org> Cc: Eric Dumazet <dada1@...mosbay.com>, dipankar@...ibm.com, linux-input@...r.kernel.org, dmitry.torokhov@...il.com, linux-kernel@...r.kernel.org Subject: Re: Question about usage of RCU in the input layer On Sat, Mar 21, 2009 at 09:51:09PM -0700, Arjan van de Ven wrote: > On Sat, 21 Mar 2009 21:38:38 -0700 > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote: > > > On Sat, Mar 21, 2009 at 08:40:45PM -0700, Arjan van de Ven wrote: > > > On Sat, 21 Mar 2009 14:07:45 -0700 > > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote: > > > > > > > On Sat, Mar 21, 2009 at 09:26:08PM +0100, Eric Dumazet wrote: > > > > > Arjan van de Ven a écrit : > > > > > > On Fri, 20 Mar 2009 18:27:46 -0700 > > > > > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote: > > > > > > > > > > > >> On Fri, Mar 20, 2009 at 11:13:54AM -0700, Arjan van de Ven > > > > > >> wrote: > > > > > >>> On Fri, 20 Mar 2009 07:31:04 -0700 > > > > > >>> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote: > > > > > >>>>> that'd be throwing out the baby with the bathwater... I'm > > > > > >>>>> trying to use the other cpus to do some of the boot work > > > > > >>>>> (so that the total goes faster); not using the other cpus > > > > > >>>>> would be counter productive to that. (As is just sitting > > > > > >>>>> in synchronize_rcu() when the other cpu is working.. > > > > > >>>>> hence this discussion ;-) > > > > > >>>> OK, so you are definitely running multiple CPUs when the > > > > > >>>> offending synchronize_rcu() executes, then? > > > > > >>> absolutely. > > > > > >>> (and I'm using bootgraph.pl in scripts to track who's > > > > > >>> stalling etc) > > > > > >>>> If so, here are some follow-on questions: > > > > > >>>> > > > > > >>>> 1. How many synchronize_rcu() calls are you seeing > > > > > >>>> on the critical boot path > > > > > >>> I've seen only this (input) one to take a long time > > > > > >> Ouch!!! A -single- synchronize_rcu() taking a full second??? > > > > > >> That indicates breakage. > > > > > >> > > > > > >>>> and what value of HZ are you running? > > > > > >>> 1000 > > > > > >> K, in absence of readers for RCU_CLASSIC, we should see a > > > > > >> handful of milliseconds for synchronize_rcu(). > > > > > > > > > > > > I've attached an instrumented bootgraph of what is going on; > > > > > > the rcu delays are shown as red blocks inside the regular > > > > > > functions as they initialize...... > > > > > > > > > > > > (svg can be viewed with inkscape, gimp, firefox and various > > > > > > other tools) > > > > > > > > > > Interesting stuff... > > > > > > > > > > I thought you mentioned i2c drivers being source of the > > > > > udelays(), but I cant see them in this svg, unless its > > > > > async_probe_hard ? > > > > > > > > Arjan, another thought -- if the udelays() are not under > > > > rcu_read_lock(), you should be able to finesse this by using > > > > CONFIG_PREEMPT_RCU, which will happily ignore spinning CPUs as > > > > long as they are not in an RCU read-side critical section. > > > > > > I'll play with that > > > In the mean time I've reduced the "other" function's time > > > significantly; so the urgency has gone away some. > > > > Good to hear! > > > > > It's still "interesting" that even in the "there is only really one > > > thread running" case the minimum delay seems to be 2700 microseconds > > > for classic RCU. Especially during bootup that sounds a bit > > > harsh.... (since that is where many "read mostly" cases actually > > > get their modifications) > > > > OK, I'll bite... 2700 microseconds measures exactly what? > > I'm measuring the time that the following code takes: > > init_completion(&rcu.completion); > /* Will wake me after RCU finished. */ > call_rcu(&rcu.head, wakeme_after_rcu); > /* Wait for it. */ > wait_for_completion(&rcu.completion); > > in kernel/rcupdate.c:synchronize_rcu(); > (I put markings around it for bootgraph to pick up). > it looks like this: > [ 0.196157] rcu_waiting @ 1 > [ 0.198978] rcu_continuing @ 1 after 2929 usec > [ 0.199585] rcu_waiting @ 1 > [ 0.201973] rcu_continuing @ 1 after 2929 usec > [ 0.208132] rcu_waiting @ 1 > [ 0.210905] rcu_continuing @ 1 after 2633 usec > [ 0.258025] rcu_waiting @ 1 > [ 0.260910] rcu_continuing @ 1 after 2742 usec > [ 0.260988] rcu_waiting @ 1 > [ 0.263910] rcu_continuing @ 1 after 2778 usec > [ 0.263987] rcu_waiting @ 1 > [ 0.266910] rcu_continuing @ 1 after 2778 usec > [ 0.273030] rcu_waiting @ 1 > [ 0.275912] rcu_continuing @ 1 after 2738 usec > [ 0.636267] rcu_waiting @ 1 > [ 0.639531] rcu_continuing @ 1 after 3113 usec > [ 0.639611] rcu_waiting @ 1 > [ 0.642006] rcu_continuing @ 1 after 2242 usec > [ 0.642086] rcu_waiting @ 1 > [ 0.645407] rcu_continuing @ 1 after 3169 usec > [ 0.645487] rcu_waiting @ 1 > [ 0.648007] rcu_continuing @ 1 after 2361 usec > [ 1.176323] rcu_waiting @ 1 > [ 1.873021] rcu_continuing @ 1 after 680368 usec > [ 1.873108] rcu_waiting @ 1 > [ 2.046045] rcu_continuing @ 1 after 168881 usec > > ok so I was not entirely right; there's a few ones that are a bit > shorter than 2700... No, my confusion -- I misread as 2700 milliseconds rather than 2700 -microseconds-. 2700 microseconds (or 2.7 milliseconds) is in the expected range for synchronize_rcu() on an HZ=1000 system. 2.7 seconds would of course be way out of line. The last two you captured above are excessive, of course -- more than 100 milliseconds each. > > Also, "really one thread" means hardware threads or software threads? > > one software thread. (as per the bootgraph) OK, then the commit called out below would not help you much anyway. > > If the former, exactly which kernel are you using? The single-CPU > > optimization was added in 2.6.29-rc7, commit ID a682604838. > > a bit after -rc8, specifically commit > 5bee17f18b595937e6beafeee5197868a3f74a06 How many synchronize_rcu() calls are you seeing on the boot path? Also, are you running with NO_HZ=y? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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