Re: [PATCH v3] memcg: expose socket memory pressure in a cgroup

[Kuniyuki Iwashima <kuniyu@google.com>]

On Tue, Jul 22, 2025 at 1:11 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Tue, Jul 22, 2025 at 12:58:17PM -0700, Kuniyuki Iwashima wrote:
> > On Tue, Jul 22, 2025 at 12:05 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> > > On Tue, Jul 22, 2025 at 11:27:39AM -0700, Kuniyuki Iwashima wrote:
> > > > On Tue, Jul 22, 2025 at 10:50 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > > > >
> > > > > On Tue, Jul 22, 2025 at 10:57:31AM +0200, Michal Koutný wrote:
> > > > > > Hello Daniel.
> > > > > >
> > > > > > On Tue, Jul 22, 2025 at 09:11:46AM +0200, Daniel Sedlak <daniel.sedlak@cdn77.com> wrote:
> > > > > > >   /sys/fs/cgroup/**/<cgroup name>/memory.net.socket_pressure
> > > > > > >
> > > > > > > The output value is an integer matching the internal semantics of the
> > > > > > > struct mem_cgroup for socket_pressure. It is a periodic re-arm clock,
> > > > > > > representing the end of the said socket memory pressure, and once the
> > > > > > > clock is re-armed it is set to jiffies + HZ.
> > > > > >
> > > > > > I don't find it ideal to expose this value in its raw form that is
> > > > > > rather an implementation detail.
> > > > > >
> > > > > > IIUC, the information is possibly valid only during one jiffy interval.
> > > > > > How would be the userspace consuming this?
> > > > > >
> > > > > > I'd consider exposing this as a cummulative counter in memory.stat for
> > > > > > simplicity (or possibly cummulative time spent in the pressure
> > > > > > condition).
> > > > > >
> > > > > > Shakeel, how useful is this vmpressure per-cgroup tracking nowadays? I
> > > > > > thought it's kind of legacy.
> > > > >
> > > > >
> > > > > Yes vmpressure is legacy and we should not expose raw underlying number
> > > > > to the userspace. How about just 0 or 1 and use
> > > > > mem_cgroup_under_socket_pressure() underlying? In future if we change
> > > > > the underlying implementation, the output of this interface should be
> > > > > consistent.
> > > >
> > > > But this is available only for 1 second, and it will not be useful
> > > > except for live debugging ?
> > >
> > > 1 second is the current implementation and it can be more if the memcg
> > > remains in memory pressure. Regarding usefullness I think the periodic
> > > stat collectors (like cadvisor or Google's internal borglet+rumbo) would
> > > be interested in scraping this interface.
> >
> > I think the cumulative counter suggested above is better at least.
>
> It is tied to the underlying implementation. If we decide to use, for
> example, PSI in future, what should this interface show?

Sorry, I'm not yet familiar with PSI so can't say what would be
especially useful.