Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcingdelay from HZ
From: Paul E. McKenney
Date: Sun Apr 14 2013 - 02:10:20 EST
On Sat, Apr 13, 2013 at 03:09:43PM -0700, Paul E. McKenney wrote:
> On Sat, Apr 13, 2013 at 12:53:36PM -0700, Josh Triplett wrote:
> > 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@xxxxxxxxxxxxxxxxxx>
> > > > > > >
> > > > > > > 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@xxxxxxxxxxx>
> > > > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > > > >
> > > > > > 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.
> FQS is RCU's idle==quiesced logic. ;-)
> In theory, RCU could add logic at idle entry to report a quiescent state,
> in fact CONFIG_RCU_FAST_NO_HZ used to do exactly that. In practice,
> this is not good for energy efficiency at runtime for a goodly number
> of workloads, which is why CONFIG_RCU_FAST_NO_HZ now relies on callback
> numbering and FQS.
> I understand that at boot time, energy efficiency is best served by
> making boot go faster, but that means that something has to tell RCU
> when boot is complete.
> > > 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?
> That is exactly the question. After all, if RCU is going to do something
> special during boot, it needs to know when boot ends. People normally
> count boot as up to user login, but RCU currently has no way to know
> when this is, at least as far as I know. Which is why I suggested that
> something tell RCU via sysfs.
> Regardless, for the usual definition of "boot is complete", user space has
> to decide when boot is complete. The kernel is out of the loop early on.
> > In any case, I don't think it makes sense to do this with FQS.
> OK, let's go through the possibilities I can imagine at the moment:
> 1. Force the scheduling-clock interrupt to remain on during
> boot. This way, each CPU could tell RCU of its idle/non-idle
> state. Of course, something then needs to tell the kernel
> when boot is over so that it can go back to energy-efficient
> 2. Set rcutree.jiffies_till_first_fqs=0 at boot time, then when
> boot is complete, set it to 3 via sysfs, or to some magic number
> telling RCU to recompute the default. This has the virtue of
> allowing different userspaces to handle this differently.
> 3. Take a half-step by having RCU register a callback during the
> latest phase of kernel-visible boot. I am under the impression
> that this is a relatively small fraction of boot, so it would
> be sub-optimal.
> 4. Make CPUs announce quiescence on each entry to idle. This
> covers the transition to idle, but when a given CPU stays idle
> for more than one grace period, RCU has to do something to verify
> that the CPU remains idle. Right now, that is FQS's job --
> it cycles through the dyntick-idle structures of all CPUs that
> have not already announced quiescence.
> 5. Make CPUs IPI RCU's grace-period kthread on each transition
> to and from idle. I might be missing something, but given the
> cost and disuptiveness of IPIs, this does not seem to me to be
> a strategy to win.
> 6. IPI the CPUs to see if they are still idle. This would defeat
> energy efficiency. Of course, RCU could take this approach
> only during boot, but it is cheaper and faster to just check
> each CPU's rcu_dynticks structure -- which is what FQS does.
> 7. Treat all normal grace periods as expedited grace periods, but
> only during boot. It is fairly easy for RCU to do this, but
> again, something has to tell RCU when boot is complete.
> 8. Your idea here. Plus more of mine as I remember them. ;-)
> So, what am I missing?
Hmmm... I suppose I could have RCU define boot as being (say) the ten
seconds following the early_inits. That is crude enough that it might
actually work reasonably well.
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