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

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Message-ID: <5352535D.8030407@cn.fujitsu.com>
Date:	Sat, 19 Apr 2014 18:43:41 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
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

On 04/19/2014 01:17 AM, Tejun Heo wrote:
> On Fri, Apr 18, 2014 at 08:49:53PM +0800, Lai Jiangshan wrote:
>> The ida callers always calls ida_pre_get() before ida_get_new*().
>> ida_pre_get() will do layer allocation, and ida_get_new*() will do layer removal.
>>
>> It causes an unneeded ping-pong. The speculative layer removal in
>> ida_get_new*() can't result expected optimization.
>>
>> So we remove the unneeded layer removal in ida_get_new*().
> 
> But the as comment says, ida doesn't want to keep extra layers around
> as it wants to keep its memory footprint minimal.  

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.


> 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.

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.

Any thought?

But we need to remove this unneeded ping-pong despite of any plan.

Thanks,
Lai
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