linux-kernel - Re: [PATCH v6 5/6] lib/dlock-list: Enable faster lookup with hashing

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Message-ID: <20171005091406.GD28132@quack2.suse.cz>
Date:   Thu, 5 Oct 2017 11:14:06 +0200
From:   Jan Kara <jack@...e.cz>
To:     Waiman Long <longman@...hat.com>
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>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] lib/dlock-list: Enable faster lookup with hashing

On Wed 04-10-17 17:20:06, Waiman Long wrote:
> Insertion and deletion is relatively cheap and mostly contention
> free for dlock-list. Lookup, on the other hand, can be rather costly
> because all the lists in a dlock-list will have to be iterated.
> 
> Currently dlock-list insertion is based on the cpu that the task is
> running on. So a given object can be inserted into any one of the
> lists depending on what the current cpu is.
> 
> 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.
> 
> The new APIs are:
> 
> struct dlock_list_head *dlock_list_hash(struct dlock_list_heads *, void *);
> void dlock_list_add(struct dlock_list_node *, struct dlock_list_head *);
> 
> Signed-off-by: Waiman Long <longman@...hat.com>

OK, this makes sense but do you have any particular user in mind? In
particular I'm not sure how big advantage this API brings over an existing
one in include/linux/list_bl.h. Sure it's a tradeoff between bitlock /
spinlock but is there a user where it matters?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR