Re: [PATCH v4] memcg: expose socket memory pressure in a cgroup

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Aug 05, 2025 at 08:44:29AM +0200, Daniel Sedlak wrote:
> This patch is a result of our long-standing debug sessions, where it all
> started as "networking is slow", and TCP network throughput suddenly
> dropped from tens of Gbps to few Mbps, and we could not see anything in
> the kernel log or netstat counters.
> 
> Currently, we have two memory pressure counters for TCP sockets [1],
> which we manipulate only when the memory pressure is signalled through
> the proto struct [2]. However, the memory pressure can also be signaled
> through the cgroup memory subsystem, which we do not reflect in the
> netstat counters. In the end, when the cgroup memory subsystem signals
> that it is under pressure, we silently reduce the advertised TCP window
> with tcp_adjust_rcv_ssthresh() to 4*advmss, which causes a significant
> throughput reduction.
> 
> Keep in mind that when the cgroup memory subsystem signals the socket
> memory pressure, it affects all sockets used in that cgroup.
> 
> This patch exposes a new file for each cgroup in sysfs which signals
> the cgroup socket memory pressure. The file is accessible in
> the following path.
> 
>   /sys/fs/cgroup/**/<cgroup name>/memory.net.socket_pressure

let's keep the name concise. Maybe memory.net.pressure?

> 
> The output value is a cumulative sum of microseconds spent
> under pressure for that particular cgroup.
> 
> Link: https://elixir.bootlin.com/linux/v6.15.4/source/include/uapi/linux/snmp.h#L231-L232 [1]
> Link: https://elixir.bootlin.com/linux/v6.15.4/source/include/net/sock.h#L1300-L1301 [2]
> Co-developed-by: Matyas Hurtik <matyas.hurtik@cdn77.com>
> Signed-off-by: Matyas Hurtik <matyas.hurtik@cdn77.com>
> Signed-off-by: Daniel Sedlak <daniel.sedlak@cdn77.com>
> ---
> Changes:
> v3 -> v4:
> - Add documentation
> - Expose pressure as cummulative counter in microseconds
> - Link to v3: https://lore.kernel.org/netdev/20250722071146.48616-1-daniel.sedlak@cdn77.com/
> 
> v2 -> v3:
> - Expose the socket memory pressure on the cgroups instead of netstat
> - Split patch
> - Link to v2: https://lore.kernel.org/netdev/20250714143613.42184-1-daniel.sedlak@cdn77.com/
> 
> v1 -> v2:
> - Add tracepoint
> - Link to v1: https://lore.kernel.org/netdev/20250707105205.222558-1-daniel.sedlak@cdn77.com/
> 
>  Documentation/admin-guide/cgroup-v2.rst |  7 +++++++
>  include/linux/memcontrol.h              |  2 ++
>  mm/memcontrol.c                         | 15 +++++++++++++++
>  mm/vmpressure.c                         |  9 ++++++++-
>  4 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 0cc35a14afbe..c810b449fb3d 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1884,6 +1884,13 @@ The following nested keys are defined.
>  	Shows pressure stall information for memory. See
>  	:ref:`Documentation/accounting/psi.rst <psi>` for details.
>  
> +  memory.net.socket_pressure
> +	A read-only single value file showing how many microseconds
> +	all sockets within that cgroup spent under pressure.
> +
> +	Note that when the sockets are under pressure, the networking
> +	throughput can be significantly degraded.
> +
>  
>  Usage Guidelines
>  ~~~~~~~~~~~~~~~~
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 87b6688f124a..6a1cb9a99b88 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -252,6 +252,8 @@ struct mem_cgroup {
>  	 * where socket memory is accounted/charged separately.
>  	 */
>  	unsigned long		socket_pressure;
> +	/* exported statistic for memory.net.socket_pressure */
> +	unsigned long		socket_pressure_duration;

I think atomic_long_t would be better.

>  
>  	int kmemcg_id;
>  	/*
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 902da8a9c643..8e299d94c073 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3758,6 +3758,7 @@ static struct mem_cgroup *mem_cgroup_alloc(struct mem_cgroup *parent)
>  	INIT_LIST_HEAD(&memcg->swap_peaks);
>  	spin_lock_init(&memcg->peaks_lock);
>  	memcg->socket_pressure = jiffies;
> +	memcg->socket_pressure_duration = 0;
>  	memcg1_memcg_init(memcg);
>  	memcg->kmemcg_id = -1;
>  	INIT_LIST_HEAD(&memcg->objcg_list);
> @@ -4647,6 +4648,15 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
>  	return nbytes;
>  }
>  
> +static int memory_socket_pressure_show(struct seq_file *m, void *v)
> +{
> +	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
> +
> +	seq_printf(m, "%lu\n", READ_ONCE(memcg->socket_pressure_duration));
> +
> +	return 0;
> +}
> +
>  static struct cftype memory_files[] = {
>  	{
>  		.name = "current",
> @@ -4718,6 +4728,11 @@ static struct cftype memory_files[] = {
>  		.flags = CFTYPE_NS_DELEGATABLE,
>  		.write = memory_reclaim,
>  	},
> +	{
> +		.name = "net.socket_pressure",
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +		.seq_show = memory_socket_pressure_show,
> +	},
>  	{ }	/* terminate */
>  };
>  
> diff --git a/mm/vmpressure.c b/mm/vmpressure.c
> index bd5183dfd879..1e767cd8aa08 100644
> --- a/mm/vmpressure.c
> +++ b/mm/vmpressure.c
> @@ -308,6 +308,8 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
>  		level = vmpressure_calc_level(scanned, reclaimed);
>  
>  		if (level > VMPRESSURE_LOW) {
> +			unsigned long socket_pressure;
> +			unsigned long jiffies_diff;
>  			/*
>  			 * Let the socket buffer allocator know that
>  			 * we are having trouble reclaiming LRU pages.
> @@ -316,7 +318,12 @@ 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);
> +			socket_pressure = jiffies + HZ;
> +
> +			jiffies_diff = min(socket_pressure - READ_ONCE(memcg->socket_pressure), HZ);
> +			memcg->socket_pressure_duration += jiffies_to_usecs(jiffies_diff);

KCSAN will complain about this. I think we can use atomic_long_add() and
don't need the one with strict ordering.

> +
> +			WRITE_ONCE(memcg->socket_pressure, socket_pressure);
>  		}
>  	}
>  }
> 
> base-commit: e96ee511c906c59b7c4e6efd9d9b33917730e000
> -- 
> 2.39.5
>