Re: [PATCH v2 net-next 2/2] mm/vmpressure: add tracepoint for socket pressure detection

[Kuniyuki Iwashima <kuniyu@google.com>]

On Tue, Jul 15, 2025 at 10:17 AM Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> On Tue, Jul 15, 2025 at 12:01 AM Daniel Sedlak <daniel.sedlak@cdn77.com> wrote:
> >
> > Hi Kuniyuki,
> >
> > On 7/14/25 8:02 PM, Kuniyuki Iwashima wrote:
> > >> +TRACE_EVENT(memcg_socket_under_pressure,
> > >> +
> > >> +       TP_PROTO(const struct mem_cgroup *memcg, unsigned long scanned,
> > >> +               unsigned long reclaimed),
> > >> +
> > >> +       TP_ARGS(memcg, scanned, reclaimed),
> > >> +
> > >> +       TP_STRUCT__entry(
> > >> +               __field(u64, id)
> > >> +               __field(unsigned long, scanned)
> > >> +               __field(unsigned long, reclaimed)
> > >> +       ),
> > >> +
> > >> +       TP_fast_assign(
> > >> +               __entry->id = cgroup_id(memcg->css.cgroup);
> > >> +               __entry->scanned = scanned;
> > >> +               __entry->reclaimed = reclaimed;
> > >> +       ),
> > >> +
> > >> +       TP_printk("memcg_id=%llu scanned=%lu reclaimed=%lu",
> > >> +               __entry->id,
> > >
> > > Maybe a noob question: How can we translate the memcg ID
> > > to the /sys/fs/cgroup/... path ?
> >
> > IMO this should be really named `cgroup_id` instead of `memcg_id`, but
> > we kept the latter to keep consistency with the rest of the file.
> >
> > To find cgroup path you can use:
> > - find /sys/fs/cgroup/ -inum `memcg_id`, and it will print "path" to the
> > affected cgroup.
> > - or you can use bpftrace tracepoint hooks and there is a helper
> > function [1].
>
> Thanks, this is good to know and worth in the commit message.
>
> >
> > Or we can put the cgroup_path to the tracepoint instead of that ID, but
> > I feel it can be too much overhead, the paths can be pretty long.
>
> Agree, the ID is good enough given we can find the cgroup by oneliner.
>
> >
> > Link: https://bpftrace.org/docs/latest#functions-cgroup_path [1]
> > > It would be nice to place this patch first and the description of
> > > patch 2 has how to use the new stat with this tracepoint.
> >
> > Sure, can do that. However, I am unsure how a good idea is to
> > cross-reference commits, since each may go through a different tree
> > because each commit is for a different subsystem. They would have to go
> > through one tree, right?
>
> Right.

Sorry, I meant to say the two patches don't need to go along to a
single tree and you can post them separately as each change is
independent.

> Probably you can just assume both patches will be merged
> and post the tracepoint patch to mm ML first and then add its
> lore.kernel.org link and howto in the stat patch and post it to netdev ML.