Date:   Mon, 6 Nov 2017 14:06:37 -0500
From:   Waiman Long <longman@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <andi@...stfloor.org>,
        Dave Chinner <dchinner@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH v4] lib/dlock-list: Scale dlock_lists_empty()

On 11/06/2017 01:47 PM, Davidlohr Bueso wrote:
> Instead of the current O(N) implementation, at the cost
> of adding an atomic counter, we can convert the call to
> an atomic_read(). The counter only serves for accounting
> empty to non-empty transitions, and vice versa; therefore
> only modified twice for each of the lists during the
> lifetime of the dlock (while used).
>
> In addition, to be able to unaccount a list_del(), we
> add a dlist pointer to each head, thus minimizing the
> overall memory footprint.
>
> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
> Changes from v3:
> - s/waiters/used_lists, more doc around the counter.
> - fixed racy scenario when the list empty/non-empty
>   condition changes after taking the lock.
> - sprinkled unlikely() around all checks, these are
>   only corner cases in the lifetime of the lock.
>
> include/linux/dlock-list.h |  8 ++++++
> lib/dlock-list.c           | 67
> +++++++++++++++++++++++++++++++++++++++-------
> 2 files changed, 65 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
> index c00c7f92ada4..e18690a9bba6 100644
> --- a/include/linux/dlock-list.h
> +++ b/include/linux/dlock-list.h
> @@ -32,10 +32,18 @@
> struct dlock_list_head {
>     struct list_head list;
>     spinlock_t lock;
> +    struct dlock_list_heads *dlist;
> } ____cacheline_aligned_in_smp;
>
> +/*
> + * This is the main dlist data structure, with the array of heads
> + * and a counter that atomically tracks if any of the lists are
> + * being used. That is, empty to non-empty (and vice versa)
> + * head->list transitions.
> + */
> struct dlock_list_heads {
>     struct dlock_list_head *heads;
> +    atomic_t used_lists;
> };
>
> /*
> diff --git a/lib/dlock-list.c b/lib/dlock-list.c
> index a4ddecc01b12..a9c855d492b8 100644
> --- a/lib/dlock-list.c
> +++ b/lib/dlock-list.c
> @@ -122,8 +122,11 @@ int __alloc_dlock_list_heads(struct
> dlock_list_heads *dlist,
>
>         INIT_LIST_HEAD(&head->list);
>         head->lock = __SPIN_LOCK_UNLOCKED(&head->lock);
> +        head->dlist = dlist;
>         lockdep_set_class(&head->lock, key);
>     }
> +
> +    atomic_set(&dlist->used_lists, 0);
>     return 0;
> }
> EXPORT_SYMBOL(__alloc_dlock_list_heads);
> @@ -139,29 +142,36 @@ void free_dlock_list_heads(struct
> dlock_list_heads *dlist)
> {
>     kfree(dlist->heads);
>     dlist->heads = NULL;
> +    atomic_set(&dlist->used_lists, 0);
> }
> EXPORT_SYMBOL(free_dlock_list_heads);
>
> /**
>  * dlock_lists_empty - Check if all the dlock lists are empty
>  * @dlist: Pointer to the dlock_list_heads structure
> - * Return: true if list is empty, false otherwise.
>  *
> - * This can be a pretty expensive function call. If this function is
> required
> - * in a performance critical path, we may have to maintain a global
> count
> - * of the list entries in the global dlock_list_heads structure instead.
> + * Return: true if all dlock lists are empty, false otherwise.
>  */
> bool dlock_lists_empty(struct dlock_list_heads *dlist)
> {
> -    int idx;
> -
>     /* Shouldn't be called before nr_dlock_lists is initialized */
>     WARN_ON_ONCE(!nr_dlock_lists);
>
> -    for (idx = 0; idx < nr_dlock_lists; idx++)
> -        if (!list_empty(&dlist->heads[idx].list))
> -            return false;
> -    return true;
> +    /*
> +     * Serialize dlist->used_lists such that a 0->1 transition is not
> +     * missed by another thread checking if any of the dlock lists are
> +     * used.
> +     *
> +     * CPU0                    CPU1
> +     * dlock_list_add()                 dlock_lists_empty()
> +     *   [S] atomic_inc(used_lists);
> +     *       smp_mb__after_atomic();
> +     *                      smp_mb__before_atomic();
> +     *                      [L] atomic_read(used_lists)
> +     *       list_add()
> +     */
> +    smp_mb__before_atomic();
> +    return !atomic_read(&dlist->used_lists);
> }
> EXPORT_SYMBOL(dlock_lists_empty);
>
> @@ -177,11 +187,39 @@ void dlock_lists_add(struct dlock_list_node *node,
>              struct dlock_list_heads *dlist)
> {
>     struct dlock_list_head *head = &dlist->heads[this_cpu_read(cpu2idx)];
> +    bool list_empty_before_lock = false;
> +
> +    /*
> +     * Optimistically bump the used_lists counter _before_ taking
> +     * the head->lock such that we don't miss a thread adding itself
> +     * to a list while spinning for the lock.
> +     *
> +     * Then, after taking the lock, recheck if the empty to non-empty
> +     * transition changed and (un)account for ourselves, accordingly.
> +     * Note that all these scenarios are corner cases, and not the
> +     * common scenario, where the lists are actually populated most
> +     * of the time.
> +     */
> +    if (unlikely(list_empty_careful(&head->list))) {
> +        list_empty_before_lock = true;
> +        atomic_inc(&dlist->used_lists);
> +        smp_mb__after_atomic();
> +    }
>
>     /*
>      * There is no need to disable preemption
>      */
>     spin_lock(&head->lock);
> +
> +    if (unlikely(!list_empty_before_lock && list_empty(&head->list))) {
> +        atomic_inc(&dlist->used_lists);
> +        smp_mb__after_atomic();
> +    }
> +    if (unlikely(list_empty_before_lock && !list_empty(&head->list))) {
> +        atomic_dec(&dlist->used_lists);
> +        smp_mb__after_atomic();
> +    }
> +
>     node->head = head;
>     list_add(&node->list, &head->list);
>     spin_unlock(&head->lock);
> @@ -212,6 +250,15 @@ void dlock_lists_del(struct dlock_list_node *node)
>         spin_lock(&head->lock);
>         if (likely(head == node->head)) {
>             list_del_init(&node->list);
> +
> +            if (unlikely(list_empty(&head->list))) {
> +                struct dlock_list_heads *dlist;
> +                dlist = node->head->dlist;
> +
> +                atomic_dec(&dlist->used_lists);
> +                smp_mb__after_atomic();
> +            }
> +
>             node->head = NULL;
>             retry = false;
>         } else {


This patch looks good to me.

Acked-by: Waiman Long <longman@...hat.com>

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