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

[Kuniyuki Iwashima <kuniyu@google.com>]

On Wed, Jul 16, 2025 at 3:43 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 16 Jul 2025 04:29:12 +0000 Kuniyuki Iwashima <kuniyu@google.com> wrote:
>
> > memcg->socket_pressure is initialised with jiffies when the memcg
> > is created.
> >
> > Once vmpressure detects that the cgroup is under memory pressure,
> > the field is updated with jiffies + HZ to signal the fact to the
> > socket layer and suppress memory allocation for one second.
> >
> > Otherwise, the field is not updated.
> >
> > mem_cgroup_under_socket_pressure() uses time_before() to check if
> > jiffies is less than memcg->socket_pressure, and this has a bug on
> > 32-bit kernel.
> >
> >   if (time_before(jiffies, memcg->socket_pressure))
> >           return true;
> >
> > As time_before() casts the final result to long, the acceptable delta
> > between two timestamps is 2 ^ (BITS_PER_LONG - 1).
> >
> > On 32-bit kernel with CONFIG_HZ=1000, this is about 24 days.
> >
> >   >>> (2 ** 31) / 1000 / 60 / 60 / 24
> >   24.855134814814818
> >
> > Once 24 days have passed since the last update of socket_pressure,
> > mem_cgroup_under_socket_pressure() starts to lie until the next
> > 24 days pass.
> >
> > 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.