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>
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
Message-ID: <0db2d93f-12cd-d703-fce7-4c3b8df5bc12@virtuozzo.com>
Date:   Mon, 26 Mar 2018 18:09:35 +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: <20180324184009.dyjlt4rj4b6y6sz3@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: 26 Mar 2018 15:09:39.1787 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: c76867b8-078d-442c-4a8d-08d5932b99a4
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 0bc7f26d-0264-416e-a6fc-8352af79c58f
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB6PR0801MB1333
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.

>> +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?
 
>> +	if (!ret) {
>> +		shrinker->id = id;
>> +		bitmap_id_start = shrinker->id + 1;
>> +	}
>> +	up_write(&bitmap_rwsem);
>> +	if (ret == -EAGAIN)
>> +		goto retry;
>> +
>> +	return ret;
>> +}

Thanks,
Kirill