Re: [PATCH] memcg: Keep socket_pressure fresh on 32-bit kernel.

[Kuniyuki Iwashima <kuniyu@google.com>]

On Wed, Jul 16, 2025 at 6:02 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 16 Jul 2025 16:58:29 -0700 Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> > On Wed, Jul 16, 2025 at 4:52 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > >
> > > On Wed, 16 Jul 2025 16:13:54 -0700 Kuniyuki Iwashima <kuniyu@google.com> wrote:
> > >
> > > > > > Thus, we need to update socket_pressure to a recent timestamp
> > > > > > periodically on 32-bit kernel.
> > > > > >
> > > > > > Let's do that every 24 hours, with a variation of about 0 to 4 hours.
> > > > >
> > > > > Can't we simply convert ->socket_preesure to a 64-bit type?
> > > > > timespec64/time64_t/etc?
> > > >
> > > > I think it's doable with get_jiffies_64() & time_before64().
> > > >
> > > > My thought was a delayed work would be better than adding
> > > > two seqlock in the networking fast path.
> > >
> > > Is it on a very fast path?  mem_cgroup_under_socket_pressure() itself
> > > doesn't look very fastpath-friendly and seqcounts are fast.  Bearing in
> > > mind that this affects 32-bit machines only.
> > >
> > > If a get_jiffies_64() call is demonstrated to be a performance issue
> > > then perhaps there's something sneaky we can do along the lines of
> > > reading jiffies_64 directly then falling back to get_jiffies_64() in
> > > the rare something-went-wrong path.  Haven't thought about it :)
> > >
> > > Dunno, the proposed patch just feels like overkill for a silly
> > > 32/66-bit issue?
> >
> > Fair enough, I'll make it u64 in v2.
> >
>
> Well, please do pay attention to any performance impact.  Measurements
> with some simple microbenchmark if possible?

I don't have a real 32-bit machine so this is a result on QEMU,
but with/without the u64 jiffie patch, the time spent in
mem_cgroup_under_socket_pressure() was 1~5us and I didn't
see any measurable delta.

no patch applied:
iperf3   273 [000]   137.296248:
probe:mem_cgroup_under_socket_pressure: (c13660d0)
                c13660d1 mem_cgroup_under_socket_pressure+0x1
([kernel.kallsyms])
iperf3   273 [000]   137.296249:
probe:mem_cgroup_under_socket_pressure__return: (c13660d0 <- c1d8fd7f)
iperf3   273 [000]   137.296251:
probe:mem_cgroup_under_socket_pressure: (c13660d0)
                c13660d1 mem_cgroup_under_socket_pressure+0x1
([kernel.kallsyms])
iperf3   273 [000]   137.296253:
probe:mem_cgroup_under_socket_pressure__return: (c13660d0 <- c1d8fd7f)


u64 jiffies patch applied:
iperf3   308 [001]   330.669370:
probe:mem_cgroup_under_socket_pressure: (c12ddba0)
                c12ddba1 mem_cgroup_under_socket_pressure+0x1
([kernel.kallsyms])
iperf3   308 [001]   330.669371:
probe:mem_cgroup_under_socket_pressure__return: (c12ddba0 <- c1ce98bf)
iperf3   308 [001]   330.669382:
probe:mem_cgroup_under_socket_pressure: (c12ddba0)
                c12ddba1 mem_cgroup_under_socket_pressure+0x1
([kernel.kallsyms])
iperf3   308 [001]   330.669384:
probe:mem_cgroup_under_socket_pressure__return: (c12ddba0 <- c1ce98bf)


So, the u64 approach is good enough :)

Thanks!