linux-kernel - Re: [PATCH 6/8] idr: avoid ping-pong

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Message-ID: <20140419130112.GB3277@htj.dyndns.org>
Date:	Sat, 19 Apr 2014 09:01:12 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jean Delvare <jdelvare@...e.de>,
	Monam Agarwal <monamagarwal123@...il.com>,
	Jeff Layton <jlayton@...hat.com>,
	Andreas Gruenbacher <agruen@...bit.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH 6/8] idr: avoid ping-pong

Hello,

On Sat, Apr 19, 2014 at 06:43:41PM +0800, Lai Jiangshan wrote:
> On 04/19/2014 01:17 AM, Tejun Heo wrote:
> It only frees one layer. And the ida_pre_get() for the next ida_get_new*()
> will allocation it back again. The aim "Throw away extra resources one by one"
> can't be achieved. It can't keep its memory footprint minimal.

Isn't the point not keeping the memory around per-ida between
allocations which can be arbitrarily long?  Why isn't it achieved?

> > I think the right
> > thing to do is implementing ida_preload() which is simliar to
> > idr_preload() and do away with per-ida layer cache.
> 
> Yes and no.
> 
> We need a static private ida_preload() for ida_simple_get() only.
> 
> Because the IDA doesn't have any query-function, so IDA's own synchronization
> is enough for all use cases, IDA should off-loads the caller's
> synchronization burden.

Hmmm?  And why can't that use preload on its own?

> In my todo-list, IDA only needs the following functions. other functions
> will be deprecated and scheduled to be removed:
> 	void ida_destroy(struct ida *ida);
> 	void ida_init(struct ida *ida);
> 	int ida_simple_get(struct ida *ida, unsigned int start, unsigned int end,
> 			   gfp_t gfp_mask);
> 	void ida_simple_remove(struct ida *ida, unsigned int id);
> 
> (I don't think we need any query-function, But...)
> If in the future we need some query-functions such as:
> 	ida_is_this_id_allocated()
> 	ida_find_next_[un]allocated_id(),
> In this case, we can expose the ida_preload() and ida_alloc() at the same time that
> we introduce the query-functions.

Regardless of interface, we can still switch to per-cpu layer caching
which would make this per-ida allocation issue go away.  I'd much
prefer to see that than debating the (dis)merits of this patch.

Thanks.

-- 
tejun
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