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

From: Paul E. McKenney
Date: Mon Jun 27 2022 - 18:46:06 EST

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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@xxxxxxxxxxx>

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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