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

Joel Fernandes
2023-03-11T22:24:04Z

On Sat, Mar 11, 2023 at 12:44:53PM -0800, Paul E. McKenney wrote:
> On Sat, Mar 04, 2023 at 04:51:45AM +0000, Joel Fernandes wrote:
> > Hi Paul,
> >
> > On Fri, Mar 03, 2023 at 05:02:51PM -0800, Paul E. McKenney wrote:
> > [..]
> > > > Qiuxu also noted impressive boot-time improvements with earlier version
> > > > of patch. An excerpt from the data he shared:
>
> Now that we have the measurement methodology put to bed...
>
> [ . . . ]
>
> > > 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.
> >
> > I prefer to keep it this way as with this method, I can not only get to
> > have variable rcu_boot_end_delay via boot parameter (as in my first patch), I
> > also don't need to add a separate sysfs entry, and can just reuse
> > 'rcu_boot_end_delay' parameter, which I also had in my first patch. And
> > adding yet another sysfs parameter will actually complicate it even more and
> > add more lines of code.
> >
> > I tested difference scenarios and it works fine, though I missed that
> > mutex locking unfortunately, I did verify different test cases work as
> > expected by manual testing.
>
> Except that you don't need that extra sysfs value. You could instead use
> any of a number of state variables that tell you that early boot is done.
> If the state says early boot (as in parsing the kernel command line),
> make the code act as it does now. Otherwise, make it accept only zero.
>
> If there really is some system that wants to set one time limit via
> the kernel boot parameter and set another at some time during boot,
> there are very simple userspace facilities to make this happen.
>
> And there is also a smaller state space and less testing to be done,
> benefits which accrue on an ongoing basis.

Ok, thanks for the suggestion and I will consider it when/if posting the next
revision of this idea. I got strong pushback from Frederic, Vlad and Steven
Rostedt on doing the timeout-based thing, so currently I am analyzing the
boot process more to see if it could be optimized instead. I tend to agree
with them now also because this feature is new and there could be bugs that
this patch might hide..

thanks,

- Joel

>
> Thanx, Paul
>
> > Here are some printks and on simple testing in Qemu:
> >
> > 1. End the boot early, CONFIG is set to 120 seconds:
> > ==================================================
> > [ 1.614968] rcu_boot_end_delay = 120000
> > [ 1.617630] schedule delayed work joel
> >
> > Boot took 1.57 seconds
> > root@(none):/# cat /sys/module/rcupdate/parameters/rcu_boot_end_delay
> > 120000
> > root@(none):/#
> > root@(none):/#
> > root@(none):/# echo 0 > /sys/module/rcupdate/parameters/rcu_boot_end_delay
> > [ 10.108394] param called joel
> > [ 10.110520] sys calling boot ended
> > [ 10.112730] rcu_boot_end_delay = 0
> > [ 10.115017] boot ended joel
> > -----------------------------------------------
> >
> > 2. End the boot passing in rcupdate.rcu_boot_end_delay as 10s.
> > This should overwride the CONFIG of 120 seconds:
> > ==================================================
> > [ 1.700090] rcu_boot_end_delay = 10000
> > [ 1.702628] schedule delayed work joel
> >
> > Boot took 1.64 seconds
> >
> > root@(none):/# [ 10.414008] rcu_boot_end_delay = 10000
> > [ 10.416670] boot ended joel
> > -----------------------------------------------
> >
> > 3. Do the same thing as #2, but extend the boot via sysfs to be longer than
> > 10 seconds:
> > ==================================================
> > [ 0.060025] param called joel
> > [ 0.060026] param called too early joel
> > [ 1.663905] rcu_boot_end_delay = 10000
> > [ 1.667051] schedule delayed work joel
> >
> > Boot took 1.61 seconds
> >
> > root@(none):/#
> > root@(none):/# echo 20000 > /sys/module/rcupdate/parameters/rcu_boot_end_delay
> > [ 6.932517] param called joel
> > [ 6.934637] sys calling boot ended
> > [ 6.936845] rcu_boot_end_delay = 20000
> > [ 6.939291] schedule delayed work joel
> > root@(none):/# [ 10.389366] rcu_boot_end_delay = 20000
> > [ 10.392047] schedule delayed work joel
> > [ 20.117416] rcu_boot_end_delay = 20000
> > [ 20.120073] boot ended joel
> > -----------------------------------------------
> >
> > The debug patch is here: https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/commit/?h=rcu/lazy/postboot
> >
> > Appended is the updated v4 patch, tested as shown above, more testing is in progress.
> >
> > thanks,
> >
> > - Joel
> >
> > ---8<-----------------------
> >
> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> > Subject: [PATCH v4] rcu: Add a minimum time for marking boot as completed
> >
> > 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.
> >
> > option-prefix PATCH v4
> > option-start
> > Tested-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> >
> > diff-note-start
> > v1->v2:
> > Update some comments and description.
> > v2->v3:
> > Add sysfs param, and update with Test data.
> > v3->v4:
> > Fix locking bug found by Paul, make code more robust
> > by refactoring locking code.
> > Doc updates.
> > ---
> > .../admin-guide/kernel-parameters.txt | 15 ++++
> > cc_list | 8 ++
> > kernel/rcu/Kconfig | 21 ++++++
> > kernel/rcu/update.c | 74 ++++++++++++++++++-
> > 4 files changed, 116 insertions(+), 2 deletions(-)
> > create mode 100644 cc_list
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 2429b5e3184b..878c2780f5db 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -5085,6 +5085,21 @@
> > rcutorture.verbose= [KNL]
> > Enable additional printk() statements.
> >
> > + rcupdate.rcu_boot_end_delay= [KNL]
> > + Minimum time in milliseconds from the start of boot
> > + 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.
> > + The sysfs node can also be used to extend the delay
> > + to be larger than the default, assuming the marking
> > + of boot complete has not yet occurred.
> > +
> > rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]
> > Dump ftrace buffer after reporting RCU CPU
> > stall warning.
> > diff --git a/cc_list b/cc_list
> > new file mode 100644
> > index 000000000000..7daed4877f5a
> > --- /dev/null
> > +++ b/cc_list
> > @@ -0,0 +1,8 @@
> > +Frederic Weisbecker <frederic@kernel.org>
> > +Joel Fernandes <joel@joelfernandes.org>
> > +Lai Jiangshan <jiangshanlai@gmail.com>
> > +linux-doc@vger.kernel.org
> > +linux-kernel@vger.kernel.org
> > +"Paul E. McKenney" <paulmck@kernel.org>
> > +rcu@vger.kernel.org
> > +urezki@gmail.com
> > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > index 9071182b1284..97f68120d1c0 100644
> > --- a/kernel/rcu/Kconfig
> > +++ b/kernel/rcu/Kconfig
> > @@ -217,6 +217,27 @@ 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 from the start of boot
> > + 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 sysfs node can
> > + also be used to extend the delay to be larger than the default, assuming
> > + the marking of boot completion has not yet occurred.
> > +
> > + 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..18ed3c15e6b5 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -224,13 +224,50 @@ void rcu_unexpedite_gp(void)
> > }
> > EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
> >
> > +/*
> > + * Minimum time in milliseconds from the start boot 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. The sysfs node
> > + * can also be used to extend the delay to be larger than the default, assuming
> > + * the marking of boot complete has not yet occurred.
> > + */
> > +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);
> >
> > /*
> > - * 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)
> > +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);
> > +
> > +/* Must be called with rcu_boot_end_lock held. */
> > +static void rcu_end_inkernel_boot_locked(void)
> > {
> > + 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);
> > + return;
> > + }
> > + }
> > +
> > + cancel_delayed_work(&rcu_boot_end_work);
> > rcu_unexpedite_gp();
> > rcu_async_relax();
> > if (rcu_normal_after_boot)
> > @@ -238,6 +275,39 @@ void rcu_end_inkernel_boot(void)
> > rcu_boot_ended = true;
> > }
> >
> > +void rcu_end_inkernel_boot(void)
> > +{
> > + mutex_lock(&rcu_boot_end_lock);
> > + rcu_end_inkernel_boot_locked();
> > + mutex_unlock(&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;
> > + /*
> > + * 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) {
> > + rcu_end_inkernel_boot_locked();
> > + }
> > + 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);
> > +
> > /*
> > * Let rcutorture know when it is OK to turn it up to eleven.
> > */
> > --
> > 2.40.0.rc0.216.gc4246ad0f0-goog
> >