Re: Low TCP throughput due to vmpressure with swap enabled

Eric Dumazet
2022-12-06T19:14:08Z

On Tue, Dec 6, 2022 at 8:00 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Dec 05, 2022 at 04:50:46PM -0800, Ivan Babrou wrote:
> > And now I can see plenty of this:
> >
> > [ 108.156707][ T5175] socket pressure[2]: 4294673429
> > [ 108.157050][ T5175] socket pressure[2]: 4294673429
> > [ 108.157301][ T5175] socket pressure[2]: 4294673429
> > [ 108.157581][ T5175] socket pressure[2]: 4294673429
> > [ 108.157874][ T5175] socket pressure[2]: 4294673429
> > [ 108.158254][ T5175] socket pressure[2]: 4294673429
> >
> > I think the first result below is to blame:
> >
> > $ rg '.->socket_pressure' mm
> > mm/memcontrol.c
> > 5280: memcg->socket_pressure = jiffies;
> > 7198: memcg->socket_pressure = 0;
> > 7201: memcg->socket_pressure = 1;
> > 7211: memcg->socket_pressure = 0;
> > 7215: memcg->socket_pressure = 1;
>
> Hoo boy, that's a silly mistake indeed. Thanks for tracking it down.
>
> > While we set socket_pressure to either zero or one in
> > mem_cgroup_charge_skmem, it is still initialized to jiffies on memcg
> > creation. Zero seems like a more appropriate starting point. With that
> > change I see it working as expected with no TCP speed bumps. My
> > ebpf_exporter program also looks happy and reports zero clamps in my
> > brief testing.
>
> Excellent, now this behavior makes sense.
>
> > I also think we should downgrade socket_pressure from "unsigned long"
> > to "bool", as it only holds zero and one now.
>
> Sounds good to me!
>
> Attaching the updated patch below. If nobody has any objections, I'll
> add a proper changelog, reported-bys, sign-off etc and send it out.
>
> ---
> include/linux/memcontrol.h | 8 +++---
> include/linux/vmpressure.h | 7 ++---
> mm/memcontrol.c | 20 +++++++++----
> mm/vmpressure.c | 58 ++++++--------------------------------
> mm/vmscan.c | 15 +---------
> 5 files changed, 30 insertions(+), 78 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index e1644a24009c..ef1c388be5b3 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -283,11 +283,11 @@ struct mem_cgroup {
> atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
> atomic_long_t memory_events_local[MEMCG_NR_MEMORY_EVENTS];
>
> - unsigned long socket_pressure;
> + /* Socket memory allocations have failed */
> + bool socket_pressure;
>
> /* Legacy tcp memory accounting */
> bool tcpmem_active;
> - int tcpmem_pressure;
>
> #ifdef CONFIG_MEMCG_KMEM
> int kmemcg_id;
> @@ -1701,10 +1701,10 @@ void mem_cgroup_sk_alloc(struct sock *sk);
> void mem_cgroup_sk_free(struct sock *sk);
> static inline bool mem_cgroup_under_socket_pressure(struct mem_cgroup *memcg)
> {
> - if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg->tcpmem_pressure)
> + if (!cgroup_subsys_on_dfl(memory_cgrp_subsys) && memcg->socket_pressure)

&& READ_ONCE(memcg->socket_pressure))

> return true;
> do {
> - if (time_before(jiffies, READ_ONCE(memcg->socket_pressure)))
> + if (memcg->socket_pressure)

if (READ_ONCE(...))

> return true;
> } while ((memcg = parent_mem_cgroup(memcg)));
> return false;
> diff --git a/include/linux/vmpressure.h b/include/linux/vmpressure.h
> index 6a2f51ebbfd3..20d93de37a17 100644
> --- a/include/linux/vmpressure.h
> +++ b/include/linux/vmpressure.h
> @@ -11,9 +11,6 @@
> #include <linux/eventfd.h>
>
> struct vmpressure {
> - unsigned long scanned;
> - unsigned long reclaimed;
> -
> unsigned long tree_scanned;
> unsigned long tree_reclaimed;
> /* The lock is used to keep the scanned/reclaimed above in sync. */
> @@ -30,7 +27,7 @@ struct vmpressure {
> struct mem_cgroup;
>
> #ifdef CONFIG_MEMCG
> -extern void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
> +extern void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
> unsigned long scanned, unsigned long reclaimed);
> extern void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg, int prio);
>
> @@ -44,7 +41,7 @@ extern int vmpressure_register_event(struct mem_cgroup *memcg,
> extern void vmpressure_unregister_event(struct mem_cgroup *memcg,
> struct eventfd_ctx *eventfd);
> #else
> -static inline void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree,
> +static inline void vmpressure(gfp_t gfp, struct mem_cgroup *memcg,
> unsigned long scanned, unsigned long reclaimed) {}
> static inline void vmpressure_prio(gfp_t gfp, struct mem_cgroup *memcg,
> int prio) {}
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2d8549ae1b30..0d4b9dbe775a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5277,7 +5277,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
> vmpressure_init(&memcg->vmpressure);
> INIT_LIST_HEAD(&memcg->event_list);
> spin_lock_init(&memcg->event_list_lock);
> - memcg->socket_pressure = jiffies;
> #ifdef CONFIG_MEMCG_KMEM
> memcg->kmemcg_id = -1;
> INIT_LIST_HEAD(&memcg->objcg_list);
> @@ -7195,10 +7194,10 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages,
> struct page_counter *fail;
>
> if (page_counter_try_charge(&memcg->tcpmem, nr_pages, &fail)) {
> - memcg->tcpmem_pressure = 0;

Orthogonal to your patch, but:

Maybe avoid touching this cache line too often and use READ/WRITE_ONCE() ?

if (READ_ONCE(memcg->socket_pressure))
WRITE_ONCE(memcg->socket_pressure, false);

> + memcg->socket_pressure = false;
> return true;
> }
> - memcg->tcpmem_pressure = 1;
> + memcg->socket_pressure = true;

Same remark.

> if (gfp_mask & __GFP_NOFAIL) {
> page_counter_charge(&memcg->tcpmem, nr_pages);
> return true;
> @@ -7206,12 +7205,21 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages,
> return false;
> }
>
> - if (try_charge(memcg, gfp_mask, nr_pages) == 0) {
> - mod_memcg_state(memcg, MEMCG_SOCK, nr_pages);
> - return true;
> + if (try_charge(memcg, gfp_mask & ~__GFP_NOFAIL, nr_pages) == 0) {
> + memcg->socket_pressure = false;

same remark.

> + goto success;
> + }
> + memcg->socket_pressure = true;

same remark.

> + if (gfp_mask & __GFP_NOFAIL) {
> + try_charge(memcg, gfp_mask, nr_pages);
> + goto success;
> }
>
> return false;
> +
> +success:
> + mod_memcg_state(memcg, MEMCG_SOCK, nr_pages);
> + return true;
> }
>
> /**