Message-ID: <917bb0a2-acb5-f2af-317b-73f1fa60f22d@bytedance.com>
Date:   Thu, 16 Dec 2021 23:43:23 +0800
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Subject: Re: [PATCH 1/2] mm: add MemAvailable to per-node meminfo
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     akpm@linux-foundation.org, rafael@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, songmuchun@bytedance.com
References: <20211216124655.32247-1-zhengqi.arch@bytedance.com>
 <20211216124655.32247-2-zhengqi.arch@bytedance.com>
 <Ybs8P3WcPtShi15o@kroah.com>
 <34cc2f26-9fba-f7c6-5dae-d21498ccbe2d@bytedance.com>
 <YbtdIpJal3keEWf8@kroah.com>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <YbtdIpJal3keEWf8@kroah.com>
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On 12/16/21 11:37 PM, Greg KH wrote:
> On Thu, Dec 16, 2021 at 11:31:36PM +0800, Qi Zheng wrote:
>>
>>
>> On 12/16/21 9:16 PM, Greg KH wrote:
>>> On Thu, Dec 16, 2021 at 08:46:54PM +0800, Qi Zheng wrote:
>>>> In /proc/meminfo, we can show the sum of all the available memory
>>>> as "MemAvailable". Add the same counter also to per-node meminfo
>>>> under /sys.
>>>>
>>>> With this counter, some processes that bind nodes can make some
>>>> decisions by reading the "MemAvailable" of the corresponding nodes
>>>> directly.
>>>>
>>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>>> ---
>>>>    drivers/base/node.c    |  4 ++++
>>>>    include/linux/mm.h     |  1 +
>>>>    include/linux/mmzone.h |  5 +++++
>>>>    mm/page_alloc.c        | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
>>>>    4 files changed, 59 insertions(+)
>>>>
>>>> diff --git a/drivers/base/node.c b/drivers/base/node.c
>>>> index 87acc47e8951..deb2a7965ae4 100644
>>>> --- a/drivers/base/node.c
>>>> +++ b/drivers/base/node.c
>>>> @@ -375,8 +375,10 @@ static ssize_t node_read_meminfo(struct device *dev,
>>>>    	struct sysinfo i;
>>>>    	unsigned long sreclaimable, sunreclaimable;
>>>>    	unsigned long swapcached = 0;
>>>> +	long available;
>>>>    	si_meminfo_node(&i, nid);
>>>> +	available = si_mem_available_node(&i, nid);
>>>>    	sreclaimable = node_page_state_pages(pgdat, NR_SLAB_RECLAIMABLE_B);
>>>>    	sunreclaimable = node_page_state_pages(pgdat, NR_SLAB_UNRECLAIMABLE_B);
>>>>    #ifdef CONFIG_SWAP
>>>> @@ -386,6 +388,7 @@ static ssize_t node_read_meminfo(struct device *dev,
>>>>    			    "Node %d MemTotal:       %8lu kB\n"
>>>>    			    "Node %d MemFree:        %8lu kB\n"
>>>>    			    "Node %d MemUsed:        %8lu kB\n"
>>>> +			    "Node %d MemAvailable:   %8lu kB\n"
>>>
>>> You just changed a user/kernel api without documenting it anywhere, or
>>> ensuring that you did not just break anything.
>>
>> Hi greg k-h,
>>
>> The MemAvailable has long existed in the /proc/meminfo, it's meaning
>> has been described in the Documentation/filesystems/proc.rst. Since
>> the semantics of per-node MemAvailable has not changed, so I did not
>> add a new document description.
> 
> This is not a proc file, it is in sysfs.
> 
> And it violates all of the sysfs rules, and has all of the problems that
> proc files have.  So the worst of both worlds :(
> 
>>> Also, this api is crazy, and not ok, please never add anything new to
>>> it, it is broken as-is.
>>
>> The consideration of adding per-node MemAvailable is that some processes
>> that bind nodes need this information to do some decisions.
>>
>> Now their approach is to read other information in per-node meminfo
>> and /proc/sys/vm/watermark_scale_factor, and then approximate this
>> value. With this counter, they can directly read
>> /sys/devices/system/node/node*/meminfo to get the MemAvailable
>> information of each node.
>>
>> And MemTotal, MemFree and SReclaimable(etc.) all have corresponding
>> per-node versions, so I think that adding per-node MemAvailable might
>> also make sense. :)
> 
> Please no, I do not want new things added to this file, as you might
> break parsers of this file.

OK, Got it.

Thank,
Qi

> 
> Also, again, you did not document this at all in Documentation/ABI/ so
> for that reason alone it is not ok.
> 
> thanks,
> 
> greg k-h
> 

-- 
Thanks,
Qi