Messages in this thread | | | Date | Sat, 13 Apr 2013 12:53:36 -0700 | From | Josh Triplett <> | Subject | Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ |
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On Sat, Apr 13, 2013 at 12:34:25PM -0700, Paul E. McKenney wrote: > On Sat, Apr 13, 2013 at 11:18:00AM -0700, Josh Triplett wrote: > > On Fri, Apr 12, 2013 at 11:38:04PM -0700, Paul E. McKenney wrote: > > > On Fri, Apr 12, 2013 at 04:54:02PM -0700, Josh Triplett wrote: > > > > On Fri, Apr 12, 2013 at 04:19:13PM -0700, Paul E. McKenney wrote: > > > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > > > > > > > > > Systems with HZ=100 can have slow bootup times due to the default > > > > > three-jiffy delays between quiescent-state forcing attempts. This > > > > > commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based > > > > > on the value of HZ. However, this would break very large systems that > > > > > require more time between quiescent-state forcing attempts. This > > > > > commit therefore also ups the default delay by one jiffy for each > > > > > 256 CPUs that might be on the system (based off of nr_cpu_ids at > > > > > runtime, -not- NR_CPUS at build time). > > > > > > > > > > Reported-by: Paul Mackerras <paulus@au1.ibm.com> > > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > > > > > Something seems very wrong if RCU regularly hits the fqs code during > > > > boot; feels like there's some more straightforward solution we're > > > > missing. What causes these CPUs to fall under RCU's scrutiny during > > > > boot yet not actually hit the RCU codepaths naturally? > > > > > > The problem is that they are running HZ=100, so that RCU will often > > > take 30-60 milliseconds per grace period. At that point, you only > > > need 16-30 grace periods to chew up a full second, so it is not all > > > that hard to eat up the additional 8-12 seconds of boot time that > > > they were seeing. IIRC, UP boot was costing them 4 seconds. > > > > > > For HZ=1000, this would translate to 800ms to 1.2s, which is nowhere > > > near as annoying. > > > > That raises two questions, though. First, who calls synchronize_rcu() > > repeatedly during boot, and could they call call_rcu() instead to avoid > > blocking for an RCU grace period? Second, why does RCU need 3-6 jiffies > > to resolve a grace period during boot? That suggests that RCU doesn't > > actually resolve a grace period until the force-quiescent-state > > machinery kicks in, meaning that the normal quiescent-state mechanism > > didn't work. > > Indeed, converting synchronize_rcu() to call_rcu() might also be > helpful. The reason that RCU often does not resolve grace periods until > force_quiescent_state() is that it is often the case during boot that > all but one CPU is idle. RCU tries hard to avoid waking up idle CPUs, > so it must scan them. Scanning is relatively expensive, so there is > reason to wait.
How are those CPUs going idle without first telling RCU that they're quiesced? Seems like, during boot at least, you want RCU to use its idle==quiesced logic to proactively note continuously-quiescent states. Ideally, you should not hit the FQS code at all during boot.
> One thing that could be done would be to scan immediately during boot, > and then back off once boot has completed. Of course, RCU has no idea > when boot has completed, but one way to get this effect is to boot > with rcutree.jiffies_till_first_fqs=0, and then use sysfs to set it > to 3 once boot has completed.
What do you mean by "boot has completed" here? The kernel's early initialization, the kernel's initialization up to running /sbin/init, or userspace initialization up through supporting user login?
In any case, I don't think it makes sense to do this with FQS.
- Josh Triplett
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