Message-ID: <0089986c-7276-cb6c-6fa8-a46ebc818244@quicinc.com>
Date:   Fri, 1 Jul 2022 10:34:22 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     <paulmck@...nel.org>
CC:     <frederic@...nel.org>, <josh@...htriplett.org>,
        <rostedt@...dmis.org>, <mathieu.desnoyers@...icios.com>,
        <jiangshanlai@...il.com>, <joel@...lfernandes.org>,
        <linux-kernel@...r.kernel.org>, <zhangfei.gao@...mail.com>,
        <boqun.feng@...il.com>, <urezki@...il.com>,
        <shameerali.kolothum.thodi@...wei.com>, <pbonzini@...hat.com>,
        <mtosatti@...hat.com>, <eric.auger@...hat.com>,
        <chenxiang66@...ilicon.com>, <maz@...nel.org>,
        <zhangfei.gao@...aro.org>, <rcu@...r.kernel.org>
Subject: Re: [PATCH v3] srcu: Reduce blocking agressiveness of expedited grace
 periods further



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@...cinc.com>
>> Tested-by: Marc Zyngier <maz@...nel.org>
>> Tested-by: Zhangfei Gao <zhangfei.gao@...aro.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@...cinc.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"  ?


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@...aro.org>
>      Reported-by: yueluck <yueluck@....com>
>      Signed-off-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
>      Tested-by: Marc Zyngier <maz@...nel.org>
>      Tested-by: Zhangfei Gao <zhangfei.gao@...aro.org>
>      Link: https://lore.kernel.org/all/20615615-0013-5adc-584f-2b1d5c03ebfc@linaro.org/
>      Signed-off-by: Paul E. McKenney <paulmck@...nel.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);

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