Date:   Mon, 27 Jun 2022 21:26:06 -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 Tue, Jun 28, 2022 at 08:45:50AM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 6/28/2022 4:15 AM, Paul E. McKenney wrote:
> > 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?
> > 
> 
> Will fix in next version
> 
> > > 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?
> > 
> 
> oops, will fix.
> 
> > > 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.
> 
> Thanks!
> 
> > 
> > > ---
> > >   .../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?
> > 
> 
> When I use clamp() or clamp_t(), I get warning like below:
> 
> 
> CC      kernel/rcu/srcutree.o
> In file included from ./include/linux/kernel.h:26:0,
>                  from ./arch/x86/include/asm/percpu.h:27,
>                  from ./arch/x86/include/asm/current.h:6,
>                  from ./include/linux/mutex.h:14,
>                  from kernel/rcu/srcutree.c:19:
> ./include/linux/minmax.h:30:50: error: braced-group within expression
> allowed only inside a function
>  #define __cmp_once(x, y, unique_x, unique_y, op) ({ \
>                                                   ^
> ./include/linux/minmax.h:20:21: note: in definition of macro ‘__typecheck’
>   (!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
> 
> 
> Which seems to be coming from braced usage in __cmp_once()
> #define __cmp_once(x, y, unique_x, unique_y, op) ({     \
>                 typeof(x) unique_x = (x);               \
>                 typeof(y) unique_y = (y);               \
>                 __cmp(unique_x, unique_y, op); })
> 
> ... as I am using clamp() to initialize module param , which is outside of a
> function.
> 
> static ulong srcu_max_nodelay_phase = SRCU_DEFAULT_MAX_NODELAY_PHASE;

OK, so the type-checking is indeed causing you trouble.  Hey, I had to ask!

							Thanx, Paul

> Thanks
> Neeraj
> 
> > 							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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