linux-kernel - Re: [PATCH 2/2] kfree_rcu() should use kfree

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Date:   Wed, 4 Apr 2018 00:16:06 -0700
From:   Rao Shoaib <rao.shoaib@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
        joe@...ches.com, brouer@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH 2/2] kfree_rcu() should use kfree_bulk() interface



On 04/03/2018 07:23 PM, Matthew Wilcox wrote:
> On Tue, Apr 03, 2018 at 05:55:55PM -0700, Rao Shoaib wrote:
>> On 04/03/2018 01:58 PM, Matthew Wilcox wrote:
>>> I think you might be better off with an IDR.  The IDR can always
>>> contain one entry, so there's no need for this 'rbf_list_head' or
>>> __rcu_bulk_schedule_list.  The IDR contains its first 64 entries in
>>> an array (if that array can be allocated), so it's compatible with the
>>> kfree_bulk() interface.
>>>
>> I have just familiarized myself with what IDR is by reading your article. If
>> I am incorrect please correct me.
>>
>> The list and head you have pointed are only used  if the container can not
>> be allocated. That could happen with IDR as well. Note that the containers
>> are allocated at boot time and are re-used.
> No, it can't happen with the IDR.  The IDR can always contain one entry
> without allocating anything.  If you fail to allocate the second entry,
> just free the first entry.
>
>> IDR seems to have some overhead, such as I have to specifically add the
>> pointer and free the ID, plus radix tree maintenance.
> ... what?  Adding a pointer is simply idr_alloc(), and you get back an
> integer telling you which index it has.  Your data structure has its
> own set of overhead.
The only overhead is a pointer that points to the head and an int to 
keep count. If I use idr, I would have to allocate an struct idr which 
is much larger. idr_alloc()/idr_destroy() operations are much more 
costly than updating two pointers. As the pointers are stored in 
slots/nodes corresponding to the id, I would  have to retrieve the 
pointers by calling idr_remove() to pass them to be freed, the 
slots/nodes would constantly be allocated and freed.

IDR is a very useful interface for allocating/managing ID's but I really 
do not see the justification for using it over here, perhaps you can 
elaborate more on the benefits and also on how I can just pass the array 
to be freed.

Shoaib