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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.
We don't need to worry about this on 64-bit machines unless they serve for
300 million years.
>>> (2 ** 63) / 1000 / 60 / 60 / 24 / 365
292471208.6775361
Let's convert memcg->socket_pressure to u64.
Performance teting:
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.
Link: https://lkml.kernel.org/r/20250717194645.1096500-1-kuniyu@google.com
Fixes: 8e8ae64524 ("mm: memcontrol: hook up vmpressure to socket pressure")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reported-by: Neal Cardwell <ncardwell@google.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <ncardwell@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.