linux-kernel - Re: [PATCH v8 1/6] lib/dlock-list: Distributed and lock-protected lists

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Date:   Thu, 2 Nov 2017 13:30:53 -0400
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 v8 1/6] lib/dlock-list: Distributed and lock-protected
 lists

On 11/02/2017 01:04 PM, Davidlohr Bueso wrote:
> On Tue, 31 Oct 2017, Waiman Long wrote:
>
>> +/**
>> + * 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.
>> + */
>
> I vote for doing this in the original version. How about the following?
>
>> +bool dlock_lists_empty(struct dlock_list_heads *dlist)
>> +{
>> +    int idx;
>> +
>> +    for (idx = 0; idx < nr_cpu_ids; idx++)
>> +        if (!list_empty(&dlist->heads[idx].list))
>> +            return false;
>> +    return true;
>> +}
>> +EXPORT_SYMBOL(dlock_lists_empty);
>
> ----------8<-----------------------------------------------
> From: Davidlohr Bueso <dave@...olabs.net>
> Subject: [PATCH] lib/dlock-list: Scale dlock_lists_empty()
>
> Instead of the current O(N) implementation; at the cost
> of adding an atomic counter. We also need to add a heads
> pointer to the node structure such that we can unaccount
> a thread doing list_del().
>

The counter will then become the single contention point for all
concurrent updates to the dlock-list. So it will have a big impact on
performance. On the other hand, instead of being a counter of # of
items, we can make that a counter of # of non-empty lists. So its value
will only be changed when a list go from empty to non-empty and vice
versa. That will greatly reduce the number of updates to that counter.


> Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
> ---
> include/linux/dlock-list.h |  2 ++
> lib/dlock-list.c           | 40 ++++++++++++++++++++++++++++------------
> 2 files changed, 30 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/dlock-list.h b/include/linux/dlock-list.h
> index c00c7f92ada4..dd73d5787885 100644
> --- a/include/linux/dlock-list.h
> +++ b/include/linux/dlock-list.h
> @@ -36,6 +36,7 @@ struct dlock_list_head {
>
> struct dlock_list_heads {
>     struct dlock_list_head *heads;
> +    atomic_t waiters;
> };
>
> /*
> @@ -44,6 +45,7 @@ struct dlock_list_heads {
> struct dlock_list_node {
>     struct list_head list;
>     struct dlock_list_head *head;
> +    struct dlock_list_heads *heads;
> };
>

I don't want to add a new data item into dlock_list_node as there can be
thousands or even of them in the system. Instead, I prefer increasing the
size of dlock_list_head which only have a limited number of them and
they have unused space because they are cacheline aligned.

Cheers,
Longman