---

yaml
---
r: 278270
b: refs/heads/master
c: 75957ba
h: refs/heads/master
v: v3

[refs]

@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 85c10f28286148ee5cdba1d22c81936ff160596e
+refs/heads/master: 75957ba36c05b979701e9ec64b37819adc12f830

trunk/include/linux/dynamic_queue_limits.h

@@ -0,0 +1,97 @@
+/*
+ * Dynamic queue limits (dql) - Definitions
+ *
+ * Copyright (c) 2011, Tom Herbert <therbert@google.com>
+ *
+ * This header file contains the definitions for dynamic queue limits (dql).
+ * dql would be used in conjunction with a producer/consumer type queue
+ * (possibly a HW queue).  Such a queue would have these general properties:
+ *
+ *   1) Objects are queued up to some limit specified as number of objects.
+ *   2) Periodically a completion process executes which retires consumed
+ *      objects.
+ *   3) Starvation occurs when limit has been reached, all queued data has
+ *      actually been consumed, but completion processing has not yet run
+ *      so queuing new data is blocked.
+ *   4) Minimizing the amount of queued data is desirable.
+ *
+ * The goal of dql is to calculate the limit as the minimum number of objects
+ * needed to prevent starvation.
+ *
+ * The primary functions of dql are:
+ *    dql_queued - called when objects are enqueued to record number of objects
+ *    dql_avail - returns how many objects are available to be queued based
+ *      on the object limit and how many objects are already enqueued
+ *    dql_completed - called at completion time to indicate how many objects
+ *      were retired from the queue
+ *
+ * The dql implementation does not implement any locking for the dql data
+ * structures, the higher layer should provide this.  dql_queued should
+ * be serialized to prevent concurrent execution of the function; this
+ * is also true for  dql_completed.  However, dql_queued and dlq_completed  can
+ * be executed concurrently (i.e. they can be protected by different locks).
+ */
+
+#ifndef _LINUX_DQL_H
+#define _LINUX_DQL_H
+
+#ifdef __KERNEL__
+
+struct dql {
+	/* Fields accessed in enqueue path (dql_queued) */
+	unsigned int	num_queued;		/* Total ever queued */
+	unsigned int	adj_limit;		/* limit + num_completed */
+	unsigned int	last_obj_cnt;		/* Count at last queuing */
+
+	/* Fields accessed only by completion path (dql_completed) */
+
+	unsigned int	limit ____cacheline_aligned_in_smp; /* Current limit */
+	unsigned int	num_completed;		/* Total ever completed */
+
+	unsigned int	prev_ovlimit;		/* Previous over limit */
+	unsigned int	prev_num_queued;	/* Previous queue total */
+	unsigned int	prev_last_obj_cnt;	/* Previous queuing cnt */
+
+	unsigned int	lowest_slack;		/* Lowest slack found */
+	unsigned long	slack_start_time;	/* Time slacks seen */
+
+	/* Configuration */
+	unsigned int	max_limit;		/* Max limit */
+	unsigned int	min_limit;		/* Minimum limit */
+	unsigned int	slack_hold_time;	/* Time to measure slack */
+};
+
+/* Set some static maximums */
+#define DQL_MAX_OBJECT (UINT_MAX / 16)
+#define DQL_MAX_LIMIT ((UINT_MAX / 2) - DQL_MAX_OBJECT)
+
+/*
+ * Record number of objects queued. Assumes that caller has already checked
+ * availability in the queue with dql_avail.
+ */
+static inline void dql_queued(struct dql *dql, unsigned int count)
+{
+	BUG_ON(count > DQL_MAX_OBJECT);
+
+	dql->num_queued += count;
+	dql->last_obj_cnt = count;
+}
+
+/* Returns how many objects can be queued, < 0 indicates over limit. */
+static inline int dql_avail(const struct dql *dql)
+{
+	return dql->adj_limit - dql->num_queued;
+}
+
+/* Record number of completed objects and recalculate the limit. */
+void dql_completed(struct dql *dql, unsigned int count);
+
+/* Reset dql state */
+void dql_reset(struct dql *dql);
+
+/* Initialize dql state */
+int dql_init(struct dql *dql, unsigned hold_time);
+
+#endif /* _KERNEL_ */
+
+#endif /* _LINUX_DQL_H */

trunk/lib/Kconfig

@@ -244,6 +244,9 @@ config CPU_RMAP
 	bool
 	depends on SMP
 
+config DQL
+	bool
+
 #
 # Netlink attribute parsing support is select'ed if needed
 #

trunk/lib/Makefile

@@ -115,6 +115,8 @@ obj-$(CONFIG_CPU_RMAP) += cpu_rmap.o
 
 obj-$(CONFIG_CORDIC) += cordic.o
 
+obj-$(CONFIG_DQL) += dynamic_queue_limits.o
+
 hostprogs-y	:= gen_crc32table
 clean-files	:= crc32table.h
 

trunk/lib/dynamic_queue_limits.c

@@ -0,0 +1,133 @@
+/*
+ * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
+ *
+ * Copyright (c) 2011, Tom Herbert <therbert@google.com>
+ */
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/ctype.h>
+#include <linux/kernel.h>
+#include <linux/dynamic_queue_limits.h>
+
+#define POSDIFF(A, B) ((A) > (B) ? (A) - (B) : 0)
+
+/* Records completed count and recalculates the queue limit */
+void dql_completed(struct dql *dql, unsigned int count)
+{
+	unsigned int inprogress, prev_inprogress, limit;
+	unsigned int ovlimit, all_prev_completed, completed;
+
+	/* Can't complete more than what's in queue */
+	BUG_ON(count > dql->num_queued - dql->num_completed);
+
+	completed = dql->num_completed + count;
+	limit = dql->limit;
+	ovlimit = POSDIFF(dql->num_queued - dql->num_completed, limit);
+	inprogress = dql->num_queued - completed;
+	prev_inprogress = dql->prev_num_queued - dql->num_completed;
+	all_prev_completed = POSDIFF(completed, dql->prev_num_queued);
+
+	if ((ovlimit && !inprogress) ||
+	    (dql->prev_ovlimit && all_prev_completed)) {
+		/*
+		 * Queue considered starved if:
+		 *   - The queue was over-limit in the last interval,
+		 *     and there is no more data in the queue.
+		 *  OR
+		 *   - The queue was over-limit in the previous interval and
+		 *     when enqueuing it was possible that all queued data
+		 *     had been consumed.  This covers the case when queue
+		 *     may have becomes starved between completion processing
+		 *     running and next time enqueue was scheduled.
+		 *
+		 *     When queue is starved increase the limit by the amount
+		 *     of bytes both sent and completed in the last interval,
+		 *     plus any previous over-limit.
+		 */
+		limit += POSDIFF(completed, dql->prev_num_queued) +
+		     dql->prev_ovlimit;
+		dql->slack_start_time = jiffies;
+		dql->lowest_slack = UINT_MAX;
+	} else if (inprogress && prev_inprogress && !all_prev_completed) {
+		/*
+		 * Queue was not starved, check if the limit can be decreased.
+		 * A decrease is only considered if the queue has been busy in
+		 * the whole interval (the check above).
+		 *
+		 * If there is slack, the amount of execess data queued above
+		 * the the amount needed to prevent starvation, the queue limit
+		 * can be decreased.  To avoid hysteresis we consider the
+		 * minimum amount of slack found over several iterations of the
+		 * completion routine.
+		 */
+		unsigned int slack, slack_last_objs;
+
+		/*
+		 * Slack is the maximum of
+		 *   - The queue limit plus previous over-limit minus twice
+		 *     the number of objects completed.  Note that two times
+		 *     number of completed bytes is a basis for an upper bound
+		 *     of the limit.
+		 *   - Portion of objects in the last queuing operation that
+		 *     was not part of non-zero previous over-limit.  That is
+		 *     "round down" by non-overlimit portion of the last
+		 *     queueing operation.
+		 */
+		slack = POSDIFF(limit + dql->prev_ovlimit,
+		    2 * (completed - dql->num_completed));
+		slack_last_objs = dql->prev_ovlimit ?
+		    POSDIFF(dql->prev_last_obj_cnt, dql->prev_ovlimit) : 0;
+
+		slack = max(slack, slack_last_objs);
+
+		if (slack < dql->lowest_slack)
+			dql->lowest_slack = slack;
+
+		if (time_after(jiffies,
+			       dql->slack_start_time + dql->slack_hold_time)) {
+			limit = POSDIFF(limit, dql->lowest_slack);
+			dql->slack_start_time = jiffies;
+			dql->lowest_slack = UINT_MAX;
+		}
+	}
+
+	/* Enforce bounds on limit */
+	limit = clamp(limit, dql->min_limit, dql->max_limit);
+
+	if (limit != dql->limit) {
+		dql->limit = limit;
+		ovlimit = 0;
+	}
+
+	dql->adj_limit = limit + completed;
+	dql->prev_ovlimit = ovlimit;
+	dql->prev_last_obj_cnt = dql->last_obj_cnt;
+	dql->num_completed = completed;
+	dql->prev_num_queued = dql->num_queued;
+}
+EXPORT_SYMBOL(dql_completed);
+
+void dql_reset(struct dql *dql)
+{
+	/* Reset all dynamic values */
+	dql->limit = 0;
+	dql->num_queued = 0;
+	dql->num_completed = 0;
+	dql->last_obj_cnt = 0;
+	dql->prev_num_queued = 0;
+	dql->prev_last_obj_cnt = 0;
+	dql->prev_ovlimit = 0;
+	dql->lowest_slack = UINT_MAX;
+	dql->slack_start_time = jiffies;
+}
+EXPORT_SYMBOL(dql_reset);
+
+int dql_init(struct dql *dql, unsigned hold_time)
+{
+	dql->max_limit = DQL_MAX_LIMIT;
+	dql->min_limit = 0;
+	dql->slack_hold_time = hold_time;
+	dql_reset(dql);
+	return 0;
+}
+EXPORT_SYMBOL(dql_init);