Re: [PATCH v3] srcu: Reduce blocking agressiveness of expedited grace periods further

Paul E. McKenney
2022-07-01T13:47:10Z

On Fri, Jul 01, 2022 at 10:34:22AM +0530, Neeraj Upadhyay wrote:
>
>
> On 7/1/2022 10:19 AM, Paul E. McKenney wrote:
> > On Fri, Jul 01, 2022 at 08:45:45AM +0530, Neeraj Upadhyay wrote:
> > > Commit 640a7d37c3f4 ("srcu: Block less aggressively for expedited
> > > grace periods") highlights a problem where aggressively blocking
> > > SRCU expedited grace periods, as was introduced in commit
> > > 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers
> > > from consuming CPU"), introduces ~2 minutes delay to the overall
> > > ~3.5 minutes boot time, when starting VMs with "-bios QEMU_EFI.fd"
> > > cmdline on qemu, which results in very high rate of memslots
> > > add/remove, which causes > ~6000 synchronize_srcu() calls for
> > > kvm->srcu SRCU instance.
> > >
> > > Below table captures the experiments done by Zhangfei Gao and Shameer
> > > to measure the boottime impact with various values of non-sleeping
> > > per phase counts, with HZ_250 and preemption enabled:
> > >
> > > +──────────────────────────+────────────────+
> > > | SRCU_MAX_NODELAY_PHASE | Boot time (s) |
> > > +──────────────────────────+────────────────+
> > > | 100 | 30.053 |
> > > | 150 | 25.151 |
> > > | 200 | 20.704 |
> > > | 250 | 15.748 |
> > > | 500 | 11.401 |
> > > | 1000 | 11.443 |
> > > | 10000 | 11.258 |
> > > | 1000000 | 11.154 |
> > > +──────────────────────────+────────────────+
> > >
> > > Analysis on the experiment results showed improved boot time
> > > with non blocking delays close to one jiffy duration. This
> > > was also seen when number of per-phase iterations were scaled
> > > to one jiffy.
> > >
> > > So, this change scales per-grace-period phase number of non-sleeping
> > > polls, such that, non-sleeping polls are done for one jiffy. In addition
> > > to this, srcu_get_delay() call in srcu_gp_end(), which is used to calculate
> > > the delay used for scheduling callbacks, is replaced with the check for
> > > expedited grace period. This is done, to schedule cbs for completed expedited
> > > grace periods immediately, which results in improved boot time seen in
> > > experiments. Testing done by Marc and Zhangfei confirms that this change recovers
> > > most of the performance degradation in boottime; for CONFIG_HZ_250 configuration,
> > > boottime improves from 3m50s to 41s on Marc's setup; and from 2m40s to ~9.7s
> > > on Zhangfei's setup.
> > >
> > > In addition to the changes to default per phase delays, this change
> > > adds 3 new kernel parameters - srcutree.srcu_max_nodelay,
> > > srcutree.srcu_max_nodelay_phase, srcutree.srcu_retry_check_delay.
> > > This allows users to configure the srcu grace period scanning delays,
> > > depending on their system configuration requirements.
> > >
> > > Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > > Tested-by: Marc Zyngier <maz@kernel.org>
> > > Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> >
> > OK, I am going to try pulling this one into the -rcu "dev" branch,
> > and thank you all!!!
> >
> > Please see below for a wordsmithed/tagged version of this commit, and
> > please let me know if I messed something up.
>
> Thanks! looks good to me; one minor nit below.
>
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 22f57fad06c5fd35cf4b80c652ebbb4dc626fdaa
> > Author: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > Date: Fri Jul 1 08:45:45 2022 +0530
> >
> > srcu: Make expedited RCU grace periods block even less frequently
> > The purpose of commit 282d8998e997 ("srcu: Prevent expedited GPs
> > and blocking readers from consuming CPU") was to prevent a long
> > series of never-blocking expedited SRCU grace periods from blocking
> > kernel-live-patching (KLP) progress. Although it was successful, it also
> > resulted in excessive boot times on certain embedded workloads running
> > under qemu with the "-bios QEMU_EFI.fd" command line. Here "excessive"
> > means increasing the boot time up into the three-to-four minute range.
> > This increase in boot time was due to the more than 6000 back-to-back
> > invocations of synchronize_rcu_expedited() within the KVM host OS, which
> > in turn resulted from qemu's emulation of a long series of MMIO accesses.
> > Commit 640a7d37c3f4 ("srcu: Block less aggressively for expedited grace
> > periods") did not significantly help this particular use case.
> > Zhangfei Gao and Shameerali Kolothum Thodi did experiments varying the
> > value of SRCU_MAX_NODELAY_PHASE with HZ=250 and with various values
> > of non-sleeping per phase counts on a system with preemption enabled,
> > and observed the following boot times:
> > +──────────────────────────+────────────────+
> > | SRCU_MAX_NODELAY_PHASE | Boot time (s) |
> > +──────────────────────────+────────────────+
> > | 100 | 30.053 |
> > | 150 | 25.151 |
> > | 200 | 20.704 |
> > | 250 | 15.748 |
> > | 500 | 11.401 |
> > | 1000 | 11.443 |
> > | 10000 | 11.258 |
> > | 1000000 | 11.154 |
> > +──────────────────────────+────────────────+
> > Analysis on the experiment results additional improvements with non
>
> Nit: "results additional improvements" -> "results show additional
> improvements" ?

Good eyes, will fix on next rebase, and thank you again!

Thanx, Paul

> Thanks
> Neeraj
>
> > blocking delays approaching one jiffy in duration. This improvement was
> > also seen when number of per-phase iterations were scaled to one jiffy.
> > This commit therefore scales per-grace-period phase number of non-sleeping
> > polls so that non-sleeping polls extend for about one jiffy. In addition,
> > the delay-calculation call to srcu_get_delay() in srcu_gp_end() is
> > replaced with a simple check for an expedited grace period. This change
> > schedules callback invocation immediately after expedited grace periods
> > complete, which results in greatly improved boot times. Testing done
> > by Marc and Zhangfei confirms that this change recovers most of the
> > performance degradation in boottime; for CONFIG_HZ_250 configuration,
> > specifically, boot times improve from 3m50s to 41s on Marc's setup;
> > and from 2m40s to ~9.7s on Zhangfei's setup.
> > In addition to the changes to default per phase delays, this
> > change adds 3 new kernel parameters - srcutree.srcu_max_nodelay,
> > srcutree.srcu_max_nodelay_phase, and srcutree.srcu_retry_check_delay.
> > This allows users to configure the srcu grace period scanning delays in
> > order to more quickly react to additional use cases.
> > Fixes: 640a7d37c3f4 ("srcu: Block less aggressively for expedited grace periods")
> > Fixes: 282d8998e997 ("srcu: Prevent expedited GPs and blocking readers from consuming CPU")
> > Reported-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> > Reported-by: yueluck <yueluck@163.com>
> > Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > Tested-by: Marc Zyngier <maz@kernel.org>
> > Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
> > Link: https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@linaro.org/
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index af647714c113d..7e34086c64f5b 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -5805,6 +5805,24 @@
> > expediting. Set to zero to disable automatic
> > expediting.
> > + srcutree.srcu_max_nodelay [KNL]
> > + Specifies the number of no-delay instances
> > + per jiffy for which the SRCU grace period
> > + worker thread will be rescheduled with zero
> > + delay. Beyond this limit, worker thread will
> > + be rescheduled with a sleep delay of one jiffy.
> > +
> > + srcutree.srcu_max_nodelay_phase [KNL]
> > + Specifies the per-grace-period phase, number of
> > + non-sleeping polls of readers. Beyond this limit,
> > + grace period worker thread will be rescheduled
> > + with a sleep delay of one jiffy, between each
> > + rescan of the readers, for a grace period phase.
> > +
> > + srcutree.srcu_retry_check_delay [KNL]
> > + Specifies number of microseconds of non-sleeping
> > + delay between each non-sleeping poll of readers.
> > +
> > srcutree.small_contention_lim [KNL]
> > Specifies the number of update-side contention
> > events per jiffy will be tolerated before
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 0db7873f4e95b..1c304fec89c02 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -511,10 +511,52 @@ static bool srcu_readers_active(struct srcu_struct *ssp)
> > return sum;
> > }
> > -#define SRCU_INTERVAL 1 // Base delay if no expedited GPs pending.
> > -#define SRCU_MAX_INTERVAL 10 // Maximum incremental delay from slow readers.
> > -#define SRCU_MAX_NODELAY_PHASE 3 // Maximum per-GP-phase consecutive no-delay instances.
> > -#define SRCU_MAX_NODELAY 100 // Maximum consecutive no-delay instances.
> > +/*
> > + * We use an adaptive strategy for synchronize_srcu() and especially for
> > + * synchronize_srcu_expedited(). We spin for a fixed time period
> > + * (defined below, boot time configurable) to allow SRCU readers to exit
> > + * their read-side critical sections. If there are still some readers
> > + * after one jiffy, we repeatedly block for one jiffy time periods.
> > + * The blocking time is increased as the grace-period age increases,
> > + * with max blocking time capped at 10 jiffies.
> > + */
> > +#define SRCU_DEFAULT_RETRY_CHECK_DELAY 5
> > +
> > +static ulong srcu_retry_check_delay = SRCU_DEFAULT_RETRY_CHECK_DELAY;
> > +module_param(srcu_retry_check_delay, ulong, 0444);
> > +
> > +#define SRCU_INTERVAL 1 // Base delay if no expedited GPs pending.
> > +#define SRCU_MAX_INTERVAL 10 // Maximum incremental delay from slow readers.
> > +
> > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_LO 3UL // Lowmark on default per-GP-phase
> > + // no-delay instances.
> > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_HI 1000UL // Highmark on default per-GP-phase
> > + // no-delay instances.
> > +
> > +#define SRCU_UL_CLAMP_LO(val, low) ((val) > (low) ? (val) : (low))
> > +#define SRCU_UL_CLAMP_HI(val, high) ((val) < (high) ? (val) : (high))
> > +#define SRCU_UL_CLAMP(val, low, high) SRCU_UL_CLAMP_HI(SRCU_UL_CLAMP_LO((val), (low)), (high))
> > +// per-GP-phase no-delay instances adjusted to allow non-sleeping poll upto
> > +// one jiffies time duration. Mult by 2 is done to factor in the srcu_get_delay()
> > +// called from process_srcu().
> > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED \
> > + (2UL * USEC_PER_SEC / HZ / SRCU_DEFAULT_RETRY_CHECK_DELAY)
> > +
> > +// Maximum per-GP-phase consecutive no-delay instances.
> > +#define SRCU_DEFAULT_MAX_NODELAY_PHASE \
> > + SRCU_UL_CLAMP(SRCU_DEFAULT_MAX_NODELAY_PHASE_ADJUSTED, \
> > + SRCU_DEFAULT_MAX_NODELAY_PHASE_LO, \
> > + SRCU_DEFAULT_MAX_NODELAY_PHASE_HI)
> > +
> > +static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE;
> > +module_param(srcu_max_nodelay_phase, ulong, 0444);
> > +
> > +// Maximum consecutive no-delay instances.
> > +#define SRCU_DEFAULT_MAX_NODELAY (SRCU_DEFAULT_MAX_NODELAY_PHASE > 100 ? \
> > + SRCU_DEFAULT_MAX_NODELAY_PHASE : 100)
> > +
> > +static ulong srcu_max_nodelay = SRCU_DEFAULT_MAX_NODELAY;
> > +module_param(srcu_max_nodelay, ulong, 0444);
> > /*
> > * Return grace-period delay, zero if there are expedited grace
> > @@ -535,7 +577,7 @@ static unsigned long srcu_get_delay(struct srcu_struct *ssp)
> > jbase += j - gpstart;
> > if (!jbase) {
> > WRITE_ONCE(ssp->srcu_n_exp_nodelay, READ_ONCE(ssp->srcu_n_exp_nodelay) + 1);
> > - if (READ_ONCE(ssp->srcu_n_exp_nodelay) > SRCU_MAX_NODELAY_PHASE)
> > + if (READ_ONCE(ssp->srcu_n_exp_nodelay) > srcu_max_nodelay_phase)
> > jbase = 1;
> > }
> > }
> > @@ -612,15 +654,6 @@ void __srcu_read_unlock(struct srcu_struct *ssp, int idx)
> > }
> > EXPORT_SYMBOL_GPL(__srcu_read_unlock);
> > -/*
> > - * We use an adaptive strategy for synchronize_srcu() and especially for
> > - * synchronize_srcu_expedited(). We spin for a fixed time period
> > - * (defined below) to allow SRCU readers to exit their read-side critical
> > - * sections. If there are still some readers after a few microseconds,
> > - * we repeatedly block for 1-millisecond time periods.
> > - */
> > -#define SRCU_RETRY_CHECK_DELAY 5
> > -
> > /*
> > * Start an SRCU grace period.
> > */
> > @@ -706,7 +739,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp
> > */
> > static void srcu_gp_end(struct srcu_struct *ssp)
> > {
> > - unsigned long cbdelay;
> > + unsigned long cbdelay = 1;
> > bool cbs;
> > bool last_lvl;
> > int cpu;
> > @@ -726,7 +759,9 @@ static void srcu_gp_end(struct srcu_struct *ssp)
> > spin_lock_irq_rcu_node(ssp);
> > idx = rcu_seq_state(ssp->srcu_gp_seq);
> > WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
> > - cbdelay = !!srcu_get_delay(ssp);
> > + if (ULONG_CMP_LT(READ_ONCE(ssp->srcu_gp_seq), READ_ONCE(ssp->srcu_gp_seq_needed_exp)))
> > + cbdelay = 0;
> > +
> > WRITE_ONCE(ssp->srcu_last_gp_end, ktime_get_mono_fast_ns());
> > rcu_seq_end(&ssp->srcu_gp_seq);
> > gpseq = rcu_seq_current(&ssp->srcu_gp_seq);
> > @@ -927,12 +962,16 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> > */
> > static bool try_check_zero(struct srcu_struct *ssp, int idx, int trycount)
> > {
> > + unsigned long curdelay;
> > +
> > + curdelay = !srcu_get_delay(ssp);
> > +
> > for (;;) {
> > if (srcu_readers_active_idx_check(ssp, idx))
> > return true;
> > - if (--trycount + !srcu_get_delay(ssp) <= 0)
> > + if ((--trycount + curdelay) <= 0)
> > return false;
> > - udelay(SRCU_RETRY_CHECK_DELAY);
> > + udelay(srcu_retry_check_delay);
> > }
> > }
> > @@ -1588,7 +1627,7 @@ static void process_srcu(struct work_struct *work)
> > j = jiffies;
> > if (READ_ONCE(ssp->reschedule_jiffies) == j) {
> > WRITE_ONCE(ssp->reschedule_count, READ_ONCE(ssp->reschedule_count) + 1);
> > - if (READ_ONCE(ssp->reschedule_count) > SRCU_MAX_NODELAY)
> > + if (READ_ONCE(ssp->reschedule_count) > srcu_max_nodelay)
> > curdelay = 1;
> > } else {
> > WRITE_ONCE(ssp->reschedule_count, 1);
> > @@ -1680,6 +1719,11 @@ static int __init srcu_bootup_announce(void)
> > pr_info("Hierarchical SRCU implementation.\n");
> > if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
> > pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff);
> > + if (srcu_retry_check_delay != SRCU_DEFAULT_RETRY_CHECK_DELAY)
> > + pr_info("\tNon-default retry check delay of %lu us.\n", srcu_retry_check_delay);
> > + if (srcu_max_nodelay != SRCU_DEFAULT_MAX_NODELAY)
> > + pr_info("\tNon-default max no-delay of %lu.\n", srcu_max_nodelay);
> > + pr_info("\tMax phase no-delay instances is %lu.\n", srcu_max_nodelay_phase);
> > return 0;
> > }
> > early_initcall(srcu_bootup_announce);