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

[Qi Zheng]

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@xxxxxxxxxxxxx>
> > ---
> >   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