From: "Huang, Ying" <ying.huang@intel.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: linux-mm@kvack.org, akpm@linux-foundation.org,
Wei Xu <weixugc@google.com>, Yang Shi <shy828301@gmail.com>,
Davidlohr Bueso <dave@stgolabs.net>,
Tim C Chen <tim.c.chen@intel.com>,
Michal Hocko <mhocko@kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Hesham Almatary <hesham.almatary@huawei.com>,
Dave Hansen <dave.hansen@intel.com>,
Jonathan Cameron <Jonathan.Cameron@huawei.com>,
Alistair Popple <apopple@nvidia.com>,
Dan Williams <dan.j.williams@intel.com>,
Johannes Weiner <hannes@cmpxchg.org>,
jvgediya.oss@gmail.com
Subject: Re: [PATCH v10 5/8] mm/demotion: Build demotion targets based on explicit memory tiers
Date: Thu, 28 Jul 2022 14:51:34 +0800 [thread overview]
Message-ID: <87y1wdn30p.fsf@yhuang6-desk2.ccr.corp.intel.com> (raw)
In-Reply-To: <87k07zrx3t.fsf@linux.ibm.com> (Aneesh Kumar K. V.'s message of "Wed, 27 Jul 2022 10:05:50 +0530")
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
> "Huang, Ying" <ying.huang@intel.com> writes:
>
>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>>
>>> On 7/26/22 1:14 PM, Huang, Ying wrote:
>>>> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
>>>>
>
> ....
>
>>>> + */
>>>>> +int next_demotion_node(int node)
>>>>> +{
>>>>> + struct demotion_nodes *nd;
>>>>> + int target;
>>>>> +
>>>>> + if (!node_demotion)
>>>>> + return NUMA_NO_NODE;
>>>>> +
>>>>> + nd = &node_demotion[node];
>>>>> +
>>>>> + /*
>>>>> + * node_demotion[] is updated without excluding this
>>>>> + * function from running.
>>>>> + *
>>>>> + * Make sure to use RCU over entire code blocks if
>>>>> + * node_demotion[] reads need to be consistent.
>>>>> + */
>>>>> + rcu_read_lock();
>>>>> + /*
>>>>> + * If there are multiple target nodes, just select one
>>>>> + * target node randomly.
>>>>> + *
>>>>> + * In addition, we can also use round-robin to select
>>>>> + * target node, but we should introduce another variable
>>>>> + * for node_demotion[] to record last selected target node,
>>>>> + * that may cause cache ping-pong due to the changing of
>>>>> + * last target node. Or introducing per-cpu data to avoid
>>>>> + * caching issue, which seems more complicated. So selecting
>>>>> + * target node randomly seems better until now.
>>>>> + */
>>>>> + target = node_random(&nd->preferred);
>>>>
>>>> In one of the most common cases, nodes_weight(&nd->preferred) == 1.
>>>> Where, get_random_int() in node_random() just wastes CPU cycles and
>>>> random entropy. So the original struct demotion_nodes implementation
>>>> appears better.
>>>>
>>>> struct demotion_nodes {
>>>> unsigned short nr;
>>>> short nodes[DEMOTION_TARGET_NODES];
>>>> };
>>>>
>>>
>>>
>>> Is that measurable difference? using nodemask_t makes it much easier with respect to
>>> implementation. IMHO if we observe the usage of node_random() to have performance impact
>>> with nodes_weight() == 1 we should fix node_random() to handle that? If you strongly
>>> feel we should fix this, i can opencode node_random to special case node_weight() == 1?
>>
>> If there's no much difference, why not just use the existing code?
>> IMHO, it's your responsibility to prove your new implementation is
>> better via numbers, for example, reduced code lines, with better or same
>> performance.
>>
>> Another policy is just to use the existing code in the first version.
>> Then change it based on measurement.
>
> One of the reason I switched to nodemask_t is to make code simpler.
> demotion target is essentially a node mask.
>
>>
>> In general, I care more about the most common cases, that is, 0 or 1
>> demotion target.
>
> How about I switch to the below opencoded version. That should take care
> of the above concern.
Per my estimation, the performance for 0 or 1 demotion target should be
OK.
And I think that you can change node_random() implementation directly.
Because it will not hurt other users too.
Best Regards,
Huang, Ying
>>
>>> - target = node_random(&nd->preferred);
>>> + node_weight = nodes_weight(nd->preferred);
>>> + switch (node_weight) {
>>> + case 0:
>>> + target = NUMA_NO_NODE;
>>> + break;
>>> + case 1:
>>> + target = first_node(nd->preferred);
>>> + break;
>>> + default:
>>> + target = bitmap_ord_to_pos(nd->preferred.bits,
>>> + get_random_int() % node_weight, MAX_NUMNODES);
>>> + break;
>>> + }
>>>
>>>
next prev parent reply other threads:[~2022-07-28 6:51 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-20 2:59 [PATCH v10 0/8] mm/demotion: Memory tiers and demotion Aneesh Kumar K.V
2022-07-20 2:59 ` [PATCH v10 1/8] mm/demotion: Add support for explicit memory tiers Aneesh Kumar K.V
2022-07-26 3:53 ` Huang, Ying
2022-07-26 11:59 ` Aneesh Kumar K V
2022-07-27 1:16 ` Huang, Ying
2022-07-28 17:23 ` Johannes Weiner
2022-07-20 2:59 ` [PATCH v10 2/8] mm/demotion: Move memory demotion related code Aneesh Kumar K.V
2022-07-20 2:59 ` [PATCH v10 3/8] mm/demotion: Add hotplug callbacks to handle new numa node onlined Aneesh Kumar K.V
2022-07-26 4:03 ` Huang, Ying
2022-07-26 12:03 ` Aneesh Kumar K V
2022-07-27 1:53 ` Huang, Ying
2022-07-27 4:38 ` Aneesh Kumar K.V
2022-07-28 6:42 ` Huang, Ying
2022-07-20 2:59 ` [PATCH v10 4/8] mm/demotion/dax/kmem: Set node's performance level to MEMTIER_PERF_LEVEL_PMEM Aneesh Kumar K.V
2022-07-21 6:07 ` kernel test robot
2022-07-25 6:37 ` Huang, Ying
2022-07-25 6:48 ` Aneesh Kumar K V
2022-07-25 8:35 ` Huang, Ying
2022-07-25 8:42 ` Aneesh Kumar K V
2022-07-26 2:13 ` Huang, Ying
2022-07-27 4:31 ` Aneesh Kumar K.V
2022-07-28 6:39 ` Huang, Ying
2022-07-20 2:59 ` [PATCH v10 5/8] mm/demotion: Build demotion targets based on explicit memory tiers Aneesh Kumar K.V
2022-07-20 3:38 ` Aneesh Kumar K.V
2022-07-21 0:02 ` kernel test robot
2022-07-26 7:44 ` Huang, Ying
2022-07-26 12:30 ` Aneesh Kumar K V
2022-07-27 1:40 ` Huang, Ying
2022-07-27 4:35 ` Aneesh Kumar K.V
2022-07-28 6:51 ` Huang, Ying [this message]
2022-08-03 3:18 ` Aneesh Kumar K.V
2022-08-04 4:19 ` Huang, Ying
2022-07-20 2:59 ` [PATCH v10 6/8] mm/demotion: Add pg_data_t member to track node memory tier details Aneesh Kumar K.V
2022-07-26 8:02 ` Huang, Ying
2022-07-20 2:59 ` [PATCH v10 7/8] mm/demotion: Demote pages according to allocation fallback order Aneesh Kumar K.V
2022-07-26 8:24 ` Huang, Ying
2022-07-20 2:59 ` [PATCH v10 8/8] mm/demotion: Update node_is_toptier to work with memory tiers Aneesh Kumar K.V
2022-07-25 8:54 ` Huang, Ying
2022-07-25 8:56 ` Aneesh Kumar K V
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