Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 01/10] mm: Assign id to every memcg-aware shrinker
To:     Vladimir Davydov <vdavydov.dev@gmail.com>
Cc:     viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
        akpm@linux-foundation.org, tglx@linutronix.de,
        pombredanne@nexb.com, stummala@codeaurora.org,
        gregkh@linuxfoundation.org, sfr@canb.auug.org.au, guro@fb.com,
        mka@chromium.org, penguin-kernel@I-love.SAKURA.ne.jp,
        chris@chris-wilson.co.uk, longman@redhat.com, minchan@kernel.org,
        hillf.zj@alibaba-inc.com, ying.huang@intel.com,
        mgorman@techsingularity.net, shakeelb@google.com, jbacik@fb.com,
        linux@roeck-us.net, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, willy@infradead.org
References: <152163840790.21546.980703278415599202.stgit@localhost.localdomain>
 <152163847740.21546.16821490541519326725.stgit@localhost.localdomain>
 <20180324184009.dyjlt4rj4b6y6sz3@esperanza>
 <0db2d93f-12cd-d703-fce7-4c3b8df5bc12@virtuozzo.com>
 <20180327091504.zcqvr3mkuznlgwux@esperanza>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <5828e99c-74d2-6208-5ec2-3361899dd36a@virtuozzo.com>
Date:   Tue, 27 Mar 2018 18:09:20 +0300
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.6.0
MIME-Version: 1.0
In-Reply-To: <20180327091504.zcqvr3mkuznlgwux@esperanza>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Received-SPF: None (protection.outlook.com: virtuozzo.com does not designate
 permitted sender hosts)
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 Mar 2018 15:09:22.8414 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 482818c5-0f27-4c0f-a689-08d593f4b98d
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 0bc7f26d-0264-416e-a6fc-8352af79c58f
X-MS-Exchange-Transport-CrossTenantHeadersStamped: HE1PR0801MB1340
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 27.03.2018 12:15, Vladimir Davydov wrote:
> On Mon, Mar 26, 2018 at 06:09:35PM +0300, Kirill Tkhai wrote:
>> Hi, Vladimir,
>>
>> thanks for your review!
>>
>> On 24.03.2018 21:40, Vladimir Davydov wrote:
>>> Hello Kirill,
>>>
>>> I don't have any objections to the idea behind this patch set.
>>> Well, at least I don't know how to better tackle the problem you
>>> describe in the cover letter. Please, see below for my comments
>>> regarding implementation details.
>>>
>>> On Wed, Mar 21, 2018 at 04:21:17PM +0300, Kirill Tkhai wrote:
>>>> The patch introduces shrinker::id number, which is used to enumerate
>>>> memcg-aware shrinkers. The number start from 0, and the code tries
>>>> to maintain it as small as possible.
>>>>
>>>> This will be used as to represent a memcg-aware shrinkers in memcg
>>>> shrinkers map.
>>>>
>>>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>>>> ---
>>>>  include/linux/shrinker.h |    1 +
>>>>  mm/vmscan.c              |   59 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>  2 files changed, 60 insertions(+)
>>>>
>>>> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
>>>> index a3894918a436..738de8ef5246 100644
>>>> --- a/include/linux/shrinker.h
>>>> +++ b/include/linux/shrinker.h
>>>> @@ -66,6 +66,7 @@ struct shrinker {
>>>>  
>>>>  	/* These are for internal use */
>>>>  	struct list_head list;
>>>> +	int id;
>>>
>>> This definition could definitely use a comment.
>>>
>>> BTW shouldn't we ifdef it?
>>
>> Ok
>>
>>>>  	/* objs pending delete, per node */
>>>>  	atomic_long_t *nr_deferred;
>>>>  };
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index 8fcd9f8d7390..91b5120b924f 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -159,6 +159,56 @@ unsigned long vm_total_pages;
>>>>  static LIST_HEAD(shrinker_list);
>>>>  static DECLARE_RWSEM(shrinker_rwsem);
>>>>  
>>>> +#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
>>>> +static DEFINE_IDA(bitmap_id_ida);
>>>> +static DECLARE_RWSEM(bitmap_rwsem);
>>>
>>> Can't we reuse shrinker_rwsem for protecting the ida?
>>
>> I think it won't be better, since we allocate memory under this semaphore.
>> After we use shrinker_rwsem, we'll have to allocate the memory with GFP_ATOMIC,
>> which does not seems good. Currently, the patchset makes shrinker_rwsem be taken
>> for a small time, just to assign already allocated memory to maps.
> 
> AFAIR it's OK to sleep under an rwsem so GFP_ATOMIC wouldn't be
> necessary. Anyway, we only need to allocate memory when we extend
> shrinker bitmaps, which is rare. In fact, there can only be a limited
> number of such calls, as we never shrink these bitmaps (which is fine
> by me).

We take bitmap_rwsem for writing to expand shrinkers maps. If we replace
it with shrinker_rwsem and the memory allocation get into reclaim, there
will be deadlock.

>>
>>>> +static int bitmap_id_start;
>>>> +
>>>> +static int alloc_shrinker_id(struct shrinker *shrinker)
>>>> +{
>>>> +	int id, ret;
>>>> +
>>>> +	if (!(shrinker->flags & SHRINKER_MEMCG_AWARE))
>>>> +		return 0;
>>>> +retry:
>>>> +	ida_pre_get(&bitmap_id_ida, GFP_KERNEL);
>>>> +	down_write(&bitmap_rwsem);
>>>> +	ret = ida_get_new_above(&bitmap_id_ida, bitmap_id_start, &id);
>>>
>>> AFAIK ida always allocates the smallest available id so you don't need
>>> to keep track of bitmap_id_start.
>>
>> I saw mnt_alloc_group_id() does the same, so this was the reason, the additional
>> variable was used. Doesn't this gives a good advise to ida and makes it find
>> a free id faster?
> 
> As Matthew pointed out, this is rather pointless.

Kirill