linux-kernel - Re: [PATCHv9 1/3] rdmacg: Added rdma cgroup controller

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Date:	Thu, 3 Mar 2016 09:51:51 +0200
From:	Haggai Eran <haggaie@...lanox.com>
To:	Parav Pandit <pandit.parav@...il.com>, Tejun Heo <tj@...nel.org>
CC:	<cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
	<lizefan@...wei.com>, Johannes Weiner <hannes@...xchg.org>,
	Doug Ledford <dledford@...hat.com>,
	Liran Liss <liranl@...lanox.com>,
	"Hefty, Sean" <sean.hefty@...el.com>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Jonathan Corbet <corbet@....net>, <james.l.morris@...cle.com>,
	<serge@...lyn.com>, Or Gerlitz <ogerlitz@...lanox.com>,
	Matan Barak <matanb@...lanox.com>, <raindel@...lanox.com>,
	<akpm@...ux-foundation.org>,
	<linux-security-module@...r.kernel.org>,
	Yishai Hadas <yishaih@...lanox.com>
Subject: Re: [PATCHv9 1/3] rdmacg: Added rdma cgroup controller

On 03/03/2016 04:49, Parav Pandit wrote:
> Hi Tejun, Haggai,
> 
> On Thu, Mar 3, 2016 at 1:28 AM, Parav Pandit <pandit.parav@...il.com> wrote:
>>>> +     rpool->refcnt--;
>>>> +     if (rpool->refcnt == 0 && rpool->num_max_cnt == pool_info->table_len) {
>>>
>>> If the caller charges 2 and then uncharges 1 two times, the refcnt
>>> underflows?  Why not just track how many usages are zero?
>>>
>> This is certainly must fix bug. Changed refcnt to usage_sum and changed to do
>> usage_sum -= num during uncharging
>> and
>> usage_sum += num during charing.
> 
> This is not sufficient as css_get() and put are done only once per
> call, which leads to similar problem as of refcnt.
Are css_get_many() and css_put_many() relevant here?

> As I think more, I realised that this particular test is missing that
> resulted in this related bug, I realize that we don't have use case to
> have "num" field from the IB stack side.
> For bulk free IB stack will have to keep track of different or same
> rdmacg returned values to call uncharge() with right number of
> resources, all of that complexity just doesn't make sense and not
> required.
> So as first step to further simplify this, I am removing "num" input
> field from charge and uncharge API.

IIRC there are no instances in the RDMA subsystem today where userspace
allocates more than one resource at a time.

Yishai, in your proposed RSS patchset did you have a verb to allocate
multiple work queues at once?

Haggai