Date:   Fri, 3 Mar 2023 17:02:51 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org, Qiuxu Zhuo <qiuxu.zhuo@intel.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-doc@vger.kernel.org, rcu@vger.kernel.org, urezki@gmail.com
Subject: Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed
Message-ID: <20230304010251.GD1301832@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20230303213851.2090365-1-joel@joelfernandes.org>
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On Fri, Mar 03, 2023 at 09:38:51PM +0000, Joel Fernandes (Google) wrote:
> On many systems, a great deal of boot (in userspace) happens after the
> kernel thinks the boot has completed. It is difficult to determine if
> the system has really booted from the kernel side. Some features like
> lazy-RCU can risk slowing down boot time if, say, a callback has been
> added that the boot synchronously depends on. Further expedited callbacks
> can get unexpedited way earlier than it should be, thus slowing down
> boot (as shown in the data below).
> 
> For these reasons, this commit adds a config option
> 'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.
> Userspace can also make RCU's view of the system as booted, by writing the
> time in milliseconds to: /sys/module/rcupdate/parameters/rcu_boot_end_delay
> Or even just writing a value of 0 to this sysfs node.
> However, under no circumstance will the boot be allowed to end earlier
> than just before init is launched.
> 
> The default value of CONFIG_RCU_BOOT_END_DELAY is chosen as 15s. This
> suites ChromeOS and also a PREEMPT_RT system below very well, which need
> no config or parameter changes, and just a simple application of this patch. A
> system designer can also choose a specific value here to keep RCU from marking
> boot completion.  As noted earlier, RCU's perspective of the system as booted
> will not be marker until at least rcu_boot_end_delay milliseconds have passed
> or an update is made via writing a small value (or 0) in milliseconds to:
> /sys/module/rcupdate/parameters/rcu_boot_end_delay.
> 
> One side-effect of this patch is, there is a risk that a real-time workload
> launched just after the kernel boots will suffer interruptions due to expedited
> RCU, which previous ended just before init was launched. However, to mitigate
> such an issue (however unlikely), the user should either tune
> CONFIG_RCU_BOOT_END_DELAY to a smaller value than 15 seconds or write a value
> of 0 to /sys/module/rcupdate/parameters/rcu_boot_end_delay, once userspace
> boots, and before launching the real-time workload.

Much better, thank you!

> Qiuxu also noted impressive boot-time improvements with earlier version
> of patch. An excerpt from the data he shared:
> 
> 1) Testing environment:
>     OS            : CentOS Stream 8 (non-RT OS)
>     Kernel     : v6.2
>     Machine : Intel Cascade Lake server (2 sockets, each with 44 logical threads)
>     Qemu  args  : -cpu host -enable-kvm, -smp 88,threads=2,sockets=2, …
> 
> 2) OS boot time definition:
>     The time from the start of the kernel boot to the shell command line
>     prompt is shown from the console. [ Different people may have
>     different OS boot time definitions. ]
> 
> 3) Measurement method (very rough method):
>     A timer in the kernel periodically prints the boot time every 100ms.
>     As soon as the shell command line prompt is shown from the console,
>     we record the boot time printed by the timer, then the printed boot
>     time is the OS boot time.
> 
> 4) Measured OS boot time (in seconds)
>    a) Measured 10 times w/o this patch:
>         8.7s, 8.4s, 8.6s, 8.2s, 9.0s, 8.7s, 8.8s, 9.3s, 8.8s, 8.3s
>         The average OS boot time was: ~8.7s
> 
>    b) Measure 10 times w/ this patch:
>         8.5s, 8.2s, 7.6s, 8.2s, 8.7s, 8.2s, 7.8s, 8.2s, 9.3s, 8.4s
>         The average OS boot time was: ~8.3s.

Unfortunately, given that a's average is within one standard deviation
of b's average, this is most definitely not statistically significant.
Especially given only ten measurements for each case -- you need *at*
*least* 24, preferably more.  Especially in this case, where you don't
really know what the underlying distribution is.

But we can apply the binomial distribution instead of the usual
normal distribution.  First, let's sort and take the medians:

a: 8.2 8.3 8.4 8.6 8.7 8.7 8.8 8.8 9.0 9.3  Median: 8.7
b: 7.6 7.8 8.2 8.2 8.2 8.2 8.4 8.5 8.7 9.3  Median: 8.2

8/10 of a's data points are greater than 0.1 more than b's median
and 8/10 of b's data points are less than 0.1 less than a's median.
What are the odds that this happens by random chance?

This is given by sum_0^2 (0.5^10 * binomial(10,i)), which is about 0.055.
This is not quite 95% confidence, so not hugely convincing, but it is at
least close.  Not that this is the confidence that (b) is 100ms faster
than (a), not just that (b) is faster than (a).

Not sure that this really carries its weight, but in contrast to the
usual statistics based on the normal distribution, it does suggest at
least a little improvement.  On the other hand, anyone who has carefully
studied nonparametric statistics probably jumped out of the boat several
paragraphs ago.  ;-)

A few more questions interspersed below.

							Thanx, Paul

> Tested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> v1->v2:
> 	Update some comments and description.
> v2->v3:
>         Add sysfs param, and update with Test data.
> 
>  .../admin-guide/kernel-parameters.txt         | 12 ++++
>  cc_list                                       |  8 +++
>  kernel/rcu/Kconfig                            | 19 ++++++
>  kernel/rcu/update.c                           | 68 ++++++++++++++++++-
>  4 files changed, 106 insertions(+), 1 deletion(-)
>  create mode 100644 cc_list
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 2429b5e3184b..611de90d9c13 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5085,6 +5085,18 @@
>  	rcutorture.verbose= [KNL]
>  			Enable additional printk() statements.
>  
> +	rcupdate.rcu_boot_end_delay= [KNL]
> +			Minimum time in milliseconds that must elapse
> +			before the boot sequence can be marked complete
> +			from RCU's perspective, after which RCU's behavior
> +			becomes more relaxed. The default value is also
> +			configurable via CONFIG_RCU_BOOT_END_DELAY.
> +			Userspace can also mark the boot as completed
> +			sooner by writing the time in milliseconds, say once
> +			userspace considers the system as booted, to:
> +			/sys/module/rcupdate/parameters/rcu_boot_end_delay
> +			Or even just writing a value of 0 to this sysfs node.

Can userspace also extend the time in this manner?  I am not too worried
either way, but it would be good to make this clear.

If userspace writes a non-zero value, is that from the current time or
from boot?

> +
>  	rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]
>  			Dump ftrace buffer after reporting RCU CPU
>  			stall warning.
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 9071182b1284..4b5ffa36cbaf 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -217,6 +217,25 @@ config RCU_BOOST_DELAY
>  
>  	  Accept the default if unsure.
>  
> +config RCU_BOOT_END_DELAY
> +	int "Minimum time before RCU may consider in-kernel boot as completed"
> +	range 0 120000
> +	default 15000
> +	help
> +	  Default value of the minimum time in milliseconds that must elapse
> +	  before the boot sequence can be marked complete from RCU's perspective,
> +	  after which RCU's behavior becomes more relaxed.
> +	  Userspace can also mark the boot as completed sooner than this default
> +	  by writing the time in milliseconds, say once userspace considers
> +	  the system as booted, to: /sys/module/rcupdate/parameters/rcu_boot_end_delay.
> +	  Or even just writing a value of 0 to this sysfs node.
> +
> +	  The actual delay for RCU's view of the system to be marked as booted can be
> +	  higher than this value if the kernel takes a long time to initialize but it
> +	  will never be smaller than this value.
> +
> +	  Accept the default if unsure.
> +
>  config RCU_EXP_KTHREAD
>  	bool "Perform RCU expedited work in a real-time kthread"
>  	depends on RCU_BOOST && RCU_EXPERT
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 19bf6fa3ee6a..93138c92136e 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -224,18 +224,84 @@ void rcu_unexpedite_gp(void)
>  }
>  EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
>  
> +/*
> + * Minimum time in milliseconds until RCU can consider in-kernel boot as
> + * completed.  This can also be tuned at runtime to end the boot earlier, by
> + * userspace init code writing the time in milliseconds (even 0) to:
> + * /sys/module/rcupdate/parameters/rcu_boot_end_delay
> + */
> +static int rcu_boot_end_delay = CONFIG_RCU_BOOT_END_DELAY;
> +
>  static bool rcu_boot_ended __read_mostly;
> +static bool rcu_boot_end_called __read_mostly;
> +static DEFINE_MUTEX(rcu_boot_end_lock);
> +
> +static int param_set_rcu_boot_end(const char *val, const struct kernel_param *kp)
> +{
> +	uint end_ms;
> +	int ret = kstrtouint(val, 0, &end_ms);
> +
> +	if (ret)
> +		return ret;
> +	WRITE_ONCE(*(uint *)kp->arg, end_ms);

Doesn't this write to rcu_boot_end_delay outside of the lock?

> +
> +	/*
> +	 * rcu_end_inkernel_boot() should be called at least once during init
> +	 * before we can allow param changes to end the boot.
> +	 */
> +	mutex_lock(&rcu_boot_end_lock);
> +	rcu_boot_end_delay = end_ms;
> +	if (!rcu_boot_ended && rcu_boot_end_called) {
> +		mutex_unlock(&rcu_boot_end_lock);
> +		rcu_end_inkernel_boot();

Temporarily dropping rcu_boot_end_lock looks like an accident waiting
to happen.

> +	}
> +	mutex_unlock(&rcu_boot_end_lock);

And dropping it twice does not seem good, either.  Or am I missing some
subtle control-flow trick?

> +	return ret;
> +}
> +
> +static const struct kernel_param_ops rcu_boot_end_ops = {
> +	.set = param_set_rcu_boot_end,
> +	.get = param_get_uint,
> +};
> +module_param_cb(rcu_boot_end_delay, &rcu_boot_end_ops, &rcu_boot_end_delay, 0644);
>  
>  /*
> - * Inform RCU of the end of the in-kernel boot sequence.
> + * Inform RCU of the end of the in-kernel boot sequence. The boot sequence will
> + * not be marked ended until at least rcu_boot_end_delay milliseconds have passed.
>   */
> +void rcu_end_inkernel_boot(void);
> +static void rcu_boot_end_work_fn(struct work_struct *work)
> +{
> +	rcu_end_inkernel_boot();
> +}
> +static DECLARE_DELAYED_WORK(rcu_boot_end_work, rcu_boot_end_work_fn);
> +
>  void rcu_end_inkernel_boot(void)
>  {
> +	mutex_lock(&rcu_boot_end_lock);
> +	rcu_boot_end_called = true;
> +
> +	if (rcu_boot_ended)
> +		return;
> +
> +	if (rcu_boot_end_delay) {
> +		u64 boot_ms = div_u64(ktime_get_boot_fast_ns(), 1000000UL);
> +
> +		if (boot_ms < rcu_boot_end_delay) {

Isn't it necessary to cancel a previously scheduled work to make sure
that the new value overrides the old one?

Mightn't this be simpler if the user was only permitted to write zero,
thus just saying "stop immediately"?  If people really need the ability
to extend or shorten the time, a patch can be produced at that point.
And then a non-zero write to the file would become legal.

> +			schedule_delayed_work(&rcu_boot_end_work,
> +					rcu_boot_end_delay - boot_ms);
> +			mutex_unlock(&rcu_boot_end_lock);
> +			return;
> +		}
> +	}
> +
> +	cancel_delayed_work(&rcu_boot_end_work);
>  	rcu_unexpedite_gp();
>  	rcu_async_relax();
>  	if (rcu_normal_after_boot)
>  		WRITE_ONCE(rcu_normal, 1);
>  	rcu_boot_ended = true;
> +	mutex_unlock(&rcu_boot_end_lock);
>  }
>  
>  /*
> -- 
> 2.40.0.rc0.216.gc4246ad0f0-goog