Commit 378bdb97 authored Jul 17, 2025 by Kuniyuki Iwashima Committed by Andrew Morton Jul 24, 2025

memcg: convert memcg->socket_pressure to u64

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: 8e8ae645 ("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>

parent a9e056de

include/linux/memcontrol.h

+41 −3

Original line number	Diff line number	Diff line
		@@ -251,8 +251,10 @@ struct mem_cgroup {
		* that this indicator should NOT be used in legacy cgroup mode
		* where socket memory is accounted/charged separately.
		*/
		unsigned long socket_pressure;

		u64 socket_pressure;
		#if BITS_PER_LONG < 64
		seqlock_t socket_pressure_seqlock;
		#endif
		int kmemcg_id;
		/*
		* memcg->objcg is wiped out as a part of the objcg repaprenting
		@@ -1602,6 +1604,42 @@ extern struct static_key_false memcg_sockets_enabled_key;
		#define mem_cgroup_sockets_enabled static_branch_unlikely(&memcg_sockets_enabled_key)
		void mem_cgroup_sk_alloc(struct sock *sk);
		void mem_cgroup_sk_free(struct sock *sk);

		#if BITS_PER_LONG < 64
		static inline void mem_cgroup_set_socket_pressure(struct mem_cgroup *memcg)
		{
		u64 val = get_jiffies_64() + HZ;
		unsigned long flags;

		write_seqlock_irqsave(&memcg->socket_pressure_seqlock, flags);
		memcg->socket_pressure = val;
		write_sequnlock_irqrestore(&memcg->socket_pressure_seqlock, flags);
		}

		static inline u64 mem_cgroup_get_socket_pressure(struct mem_cgroup *memcg)
		{
		unsigned int seq;
		u64 val;

		do {
		seq = read_seqbegin(&memcg->socket_pressure_seqlock);
		val = memcg->socket_pressure;
		} while (read_seqretry(&memcg->socket_pressure_seqlock, seq));

		return val;
		}
		#else
		static inline void mem_cgroup_set_socket_pressure(struct mem_cgroup *memcg)
		{
		WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
		}

		static inline u64 mem_cgroup_get_socket_pressure(struct mem_cgroup *memcg)
		{
		return READ_ONCE(memcg->socket_pressure);
		}
		#endif

		static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
		{
		#ifdef CONFIG_MEMCG_V1
		@@ -1609,7 +1647,7 @@ static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
		return !!memcg->tcpmem_pressure;
		#endif /* CONFIG_MEMCG_V1 */
		do {
		if (time_before(jiffies, READ_ONCE(memcg->socket_pressure)))
		if (time_before64(get_jiffies_64(), mem_cgroup_get_socket_pressure(memcg)))
		return true;
		} while ((memcg = parent_mem_cgroup(memcg)));
		return false;

mm/memcontrol.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -3754,7 +3754,10 @@ static struct mem_cgroup mem_cgroup_alloc(struct mem_cgroup parent)
		INIT_LIST_HEAD(&memcg->memory_peaks);
		INIT_LIST_HEAD(&memcg->swap_peaks);
		spin_lock_init(&memcg->peaks_lock);
		memcg->socket_pressure = jiffies;
		memcg->socket_pressure = get_jiffies_64();
		#if BITS_PER_LONG < 64
		seqlock_init(&memcg->socket_pressure_seqlock);
		#endif
		memcg1_memcg_init(memcg);
		memcg->kmemcg_id = -1;
		INIT_LIST_HEAD(&memcg->objcg_list);

mm/vmpressure.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -316,7 +316,7 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
		* asserted for a second in which subsequent
		* pressure events can occur.
		*/
		WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
		mem_cgroup_set_socket_pressure(memcg);
		}
		}
		}