Re: [PATCH 1/2] mm: add MemAvailable to per-node meminfo

[Qi Zheng <zhengqi.arch@bytedance.com>]

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.

> 
> 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. :)

Thanks,
Qi

> 
> thanks,
> 
> greg k-h
> 

-- 
Thanks,
Qi