Re: [PATCH] mm: vmscan: don't count per-node proactive reclaim as memory pressure

[Vernon Yang <vernon2gm@gmail.com>]

On Sun, Oct 04, 2026 at 11:36:15PM -0700, Andrew Morton wrote:
> On Mon,  5 Oct 2026 14:22:36 +0800 Vernon Yang <vernon2gm@gmail.com> wrote:
>
> > When the cgroup has no memory pressure at all, writing to
> > /sys/devices/system/nodeX/reclaim triggers proactive reclaim on
> > NUMA node, causing increase in the writer cgroup's memory PSI.
> >
> > Due to this reclaim is performed in the context of the write(),
> > accounted as memory pressure on the writer, like
> > commit e22c6ed90aa9 ("mm: memcontrol: don't count limit-setting reclaim
> > as memory pressure"). This is unexpected, the phenomenon resembling
> > senpai will appear again.
>
> What is this?

This is commit e22c6ed90aa9, which addresses a scenario mentioned in
memory limits of a cgroup, in detail as follows:

    Currently, this reclaim activity is accounted as memory pressure in the
    cgroup that the writer(!) belongs to.  This is unexpected.  It
    specifically causes problems for senpai
    (https://github.com/facebookincubator/senpai), which is an agent that
    routinely adjusts the memory limits and performs associated reclaim work
    in tens or even hundreds of cgroups running on the host.  The cgroup that
    senpai is running in itself will report elevated levels of memory
    pressure, even though it itself is under no memory shortage or any sort of
    distress.

This is just to explain that similar scenarios will continue to occur,
only this time it is for the /sys/devices/system/nodeX/reclaim knob.

--
Cheers,
Vernon

> > The Documentation/ABI/stable/sysfs-devices-node documentation also
> > notes that "This interface is equivalent to the memcg variant."
> >
> > This patch unifies the semantics of the memcg and node interfaces:
> > per-node proactive reclaim is no longer counted as memory pressure,
> > and the per-node proactive reclaim interface no longer produces
> > phantom pressure.
> >
> > I ran demo[1] that performs per-node proactive reclaim 10000 times
> > in qemu, writer cgroup memory.pressure as follows:
> >
> > without patch:
> >
> > some avg10=31.53 avg60=13.42 avg300=3.31 total=10602985
> > full avg10=31.53 avg60=13.42 avg300=3.31 total=10602985
> >
> > with patch:
> >
> > some avg10=9.59 avg60=3.41 avg300=0.81 total=2686221
> > full avg10=9.59 avg60=3.41 avg300=0.81 total=2686221
>