Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

From: Paul E. McKenney
Date: Sun Mar 05 2023 - 15:34:29 EST

On Sun, Mar 05, 2023 at 12:39:01PM +0100, Uladzislau Rezki wrote:
> 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.
> >
> > 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.
> >
> > Tested-by: Qiuxu Zhuo <qiuxu.zhuo@xxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> > 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.
> > +
> > 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);
> > +
> > + /*
> > + * 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();
> > + }
> > + mutex_unlock(&rcu_boot_end_lock);
> > + 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) {
> > + schedule_delayed_work(&rcu_boot_end_work,
> > + rcu_boot_end_delay - boot_ms);
> <snip>
> urezki@pc638:~/data/raid0/coding/linux-rcu.git$ git diff
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 93138c92136e..93f426f0f4ec 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -289,7 +289,7 @@ void rcu_end_inkernel_boot(void)
>
> if (boot_ms < rcu_boot_end_delay) {
> schedule_delayed_work(&rcu_boot_end_work,
> - rcu_boot_end_delay - boot_ms);
> + msecs_to_jiffies(rcu_boot_end_delay - boot_ms));
> mutex_unlock(&rcu_boot_end_lock);
> return;
> }
> urezki@pc638:~/data/raid0/coding/linux-rcu.git$
> <snip>
>
> I think you need to apply above patch. I am not sure maybe Paul
> has already mentioned about it. But just in case.

No, I did miss that one, so thank you very much for spotting it!

Thanx, Paul