linux-kernel - Re: [PATCH v7 5/6] lib/dlock-list: Enable faster lookup with hashing

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Date:   Mon, 9 Oct 2017 12:11:07 -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 v7 5/6] lib/dlock-list: Enable faster lookup with hashing

On 10/09/2017 12:03 PM, Davidlohr Bueso wrote:
> On Mon, 09 Oct 2017, Waiman Long wrote:
>
>> On 10/09/2017 09:08 AM, Davidlohr Bueso wrote:
>>> On Thu, 05 Oct 2017, Waiman Long wrote:
>>>
>>>> This patch provides an alternative way of list selection. The caller
>>>> can provide a object context which will be hashed to one of the list
>>>> in a dlock-list. The object can then be added into that particular
>>>> list. Lookup can be done by iterating elements in the provided list
>>>> only instead of all the lists in a dlock-list.
>>>
>>> Unless I'm misusing the api, I could not find a standard way of
>>> iterating a _particular_ list head (the one the dlock_list_hash()
>>> returned). This is because iterators always want the all the heads.
>>>
>>> Also, in my particular epoll case I'd need the head->lock _not_ to
>>> be dropped after the iteration, and therefore is pretty adhoc.
>>> Currently we do:
>>>
>>> dlist_for_each_entry() {
>>>     // acquire head->lock for each list
>>> }
>>> // no locks held
>>> dlist_add()
>>>
>>> I'm thinking perhaps we could have dlist_check_add() which passes a
>>> callback to ensure we want to add the node. The function could acquire
>>> the head->lock and not release it until the very end.
>>
>> With the dlock_list_hash(), dlock-list is degenerated into a pool of
>> list where one is chosen by hashing. So the regular list iteration
>> macros like list_for_each_entry() can then be used. Of course, you have
>> to explicitly do the lock and unlock operation.
>
> Right, which seemed rather asymmetric and fragile, albeit adhoc to epoll.
> Particularly not having the iter structure, makes use directly have to
> deal with the spinlock, and there goes irq awareness out the window.
>

Right. We don't need the irq safety support in this case.

>> I could also encapsulate it a bit with inlined function like
>
> Probably not worth it, unless some other use cases came up. This just
> bulks the api even more. 

I am thinking of adding only one more init API. So it is not a
significant bulking-up at all.

Cheers,
Longman