// SPDX-License-Identifier: GPL-2.0
/*
 * Dynamic byte queue limits.  See include/linux/dynamic_queue_limits.h
 *
 * Copyright (c) 2011, Tom Herbert <therbert@google.com>
 */
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/jiffies.h>
#include <linux/dynamic_queue_limits.h>
#include <linux/compiler.h>
#include <linux/export.h>
#include <trace/events/napi.h>

#define POSDIFF(A, B) ((int)((A) - (B)) > 0 ? (A) - (B) : 0)
#define AFTER_EQ(A, B) ((int)((A) - (B)) >= 0)

static void dql_check_stall(struct dql *dql, unsigned short stall_thrs)
{
        unsigned long now;

        if (!stall_thrs)
                return;

        now = jiffies;
        /* Check for a potential stall */
        if (time_after_eq(now, dql->last_reap + stall_thrs)) {
                unsigned long hist_head, t, start, end;

                /* We are trying to detect a period of at least @stall_thrs
                 * jiffies without any Tx completions, but during first half
                 * of which some Tx was posted.
                 */
dqs_again:
                hist_head = READ_ONCE(dql->history_head);
                /* pairs with smp_wmb() in dql_queued() */
                smp_rmb();

                /* Get the previous entry in the ring buffer, which is the
                 * oldest sample.
                 */
                start = (hist_head - DQL_HIST_LEN + 1) * BITS_PER_LONG;

                /* Advance start to continue from the last reap time */
                if (time_before(start, dql->last_reap + 1))
                        start = dql->last_reap + 1;

                /* Newest sample we should have already seen a completion for */
                end = hist_head * BITS_PER_LONG + (BITS_PER_LONG - 1);

                /* Shrink the search space to [start, (now - start_thrs/2)] if
                 * `end` is beyond the stall zone
                 */
                if (time_before(now, end + stall_thrs / 2))
                        end = now - stall_thrs / 2;

                /* Search for the queued time in [t, end] */
                for (t = start; time_before_eq(t, end); t++)
                        if (test_bit(t % (DQL_HIST_LEN * BITS_PER_LONG),
                                     dql->history))
                                break;

                /* Variable t contains the time of the queue */
                if (!time_before_eq(t, end))
                        goto no_stall;

                /* The ring buffer was modified in the meantime, retry */
                if (hist_head != READ_ONCE(dql->history_head))
                        goto dqs_again;

                dql->stall_cnt++;
                dql->stall_max = max_t(unsigned short, dql->stall_max, now - t);

                trace_dql_stall_detected(dql->stall_thrs, now - t,
                                         dql->last_reap, dql->history_head,
                                         now, dql->history);
        }
no_stall:
        dql->last_reap = now;
}

/* 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, completed, num_queued;
        unsigned short stall_thrs;
        bool all_prev_completed;

        num_queued = READ_ONCE(dql->num_queued);
        /* Read stall_thrs in advance since it belongs to the same (first)
         * cache line as ->num_queued. This way, dql_check_stall() does not
         * need to touch the first cache line again later, reducing the window
         * of possible false sharing.
         */
        stall_thrs = READ_ONCE(dql->stall_thrs);

        /* Can't complete more than what's in queue */
        BUG_ON(count > num_queued - dql->num_completed);

        completed = dql->num_completed + count;
        limit = dql->limit;
        ovlimit = POSDIFF(num_queued - dql->num_completed, limit);
        inprogress = num_queued - completed;
        prev_inprogress = dql->prev_num_queued - dql->num_completed;
        all_prev_completed = AFTER_EQ(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 excess data queued above
                 * 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 = READ_ONCE(dql->last_obj_cnt);
        dql->num_completed = completed;
        dql->prev_num_queued = num_queued;

        dql_check_stall(dql, stall_thrs);
}
EXPORT_SYMBOL(dql_completed);

void dql_reset(struct dql *dql)
{
        /* Reset all dynamic values */
        dql->limit = dql->min_limit;
        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;

        dql->last_reap = jiffies;
        dql->history_head = jiffies / BITS_PER_LONG;
        memset(dql->history, 0, sizeof(dql->history));
}
EXPORT_SYMBOL(dql_reset);

void dql_init(struct dql *dql, unsigned int hold_time)
{
        dql->max_limit = DQL_MAX_LIMIT;
        dql->min_limit = 0;
        dql->slack_hold_time = hold_time;
        dql->stall_thrs = 0;
        dql_reset(dql);
}
EXPORT_SYMBOL(dql_init);