srcu: Make expedited RCU grace periods block even less frequently

The purpose of commit 282d899 ("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 640a7d3 ("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 show additional improvements with
CPU-bound 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: 640a7d3 ("srcu: Block less aggressively for expedited grace periods")
Fixes: 282d899 ("srcu: Prevent expedited GPs and blocking readers from consuming CPU")
Reported-by: Zhangfei Gao <[email protected]>
Reported-by: yueluck <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>
Tested-by: Marc Zyngier <[email protected]>
Tested-by: Zhangfei Gao <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Paul E. McKenney <[email protected]>

Author: Neeraj Upadhyay

Documentation/admin-guide/kernel-parameters.txt

@@ -5771,6 +5771,24 @@
 			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

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);