Date:   Mon, 27 Jun 2022 15:45:58 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Neeraj Upadhyay <quic_neeraju@...cinc.com>
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
Subject: Re: [PATCH] srcu: Reduce blocking agressiveness of expedited grace
 periods further

On Mon, Jun 27, 2022 at 06:07:06PM +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, Shameer,

Zhangfei Gao and Shameerali Kolothum without commas?

> 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, soiuch that, non-sleeping polls are done for one jiffy. In addition

such that?

> 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.
> 
> 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>

I have queued this on an experimental branch quic_neeraju.2022.06.27a
for testing purposes.  One question below.

> ---
>  .../admin-guide/kernel-parameters.txt         | 18 +++++
>  kernel/rcu/srcutree.c                         | 79 ++++++++++++++-----
>  2 files changed, 78 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index af647714c113..7e34086c64f5 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 0db7873f4e95..006828b9c41a 100644
> --- a/kernel/rcu/srcutree.c
> +++ b/kernel/rcu/srcutree.c
> @@ -511,10 +511,49 @@ 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))

Can these just be clamp()?  Or does its type checking get in the way?

							Thanx, Paul

> +// 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_HI(SRCU_UL_CLAMP_LO(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);
> +
> +#define SRCU_DEFAULT_MAX_NODELAY	100	// Maximum consecutive no-delay instances.
> +
> +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 +574,7 @@ 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 +651,6 @@ 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 +736,7 @@ 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 +756,9 @@ 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 +959,16 @@ 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 +1624,7 @@ 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 +1716,11 @@ 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);
> -- 
> 2.17.1
> 

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