memcg: multi-memcg percpu charge cache

Memory cgroup accounting is expensive and to reduce the cost, the kernel
maintains per-cpu charge cache for a single memcg. So, if a charge
request comes for a different memcg, the kernel will flush the old memcg's
charge cache and then charge the newer memcg a fixed amount (64 pages),
subtracts the charge request amount and stores the remaining in the
per-cpu charge cache for the newer memcg.

This mechanism is based on the assumption that the kernel, for locality,
keep a process on a CPU for long period of time and most of the charge
requests from that process will be served by that CPU's local charge
cache.

However this assumption breaks down for incoming network traffic in a
multi-tenant machine. We are in the process of running multiple workloads
on a single machine and if such workloads are network heavy, we are seeing
very high network memory accounting cost. We have observed multiple CPUs
spending almost 100% of their time in net_rx_action and almost all of that
time is spent in memcg accounting of the network traffic.

More precisely, net_rx_action is serving packets from multiple workloads
and is observing/serving mix of packets of these workloads. The memcg
switch of per-cpu cache is very expensive and we are observing a lot of
memcg switches on the machine. Almost all the time is being spent on
charging new memcg and flushing older memcg cache. So, definitely we need
per-cpu cache that support multiple memcgs for this scenario.

This patch implements a simple (and dumb) multiple memcg percpu charge
cache. Actually we started with more sophisticated LRU based approach but
the dumb one was always better than the sophisticated one by 1% to 3%, so
going with the simple approach.

Some of the design choices are:

1. Fit all caches memcgs in a single cacheline.
2. The cache array can be mix of empty slots or memcg charged slots, so
the kernel has to traverse the full array.
3. The cache drain from the reclaim will drain all cached memcgs to keep
things simple.

To evaluate the impact of this optimization, on a 72 CPUs machine, we ran
the following workload where each netperf client runs in a different
cgroup. The next-20250415 kernel is used as base.

$ netserver -6
$ netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K

number of clients | Without patch | With patch
6 | 42584.1 Mbps | 48603.4 Mbps (14.13% improvement)
12 | 30617.1 Mbps | 47919.7 Mbps (56.51% improvement)
18 | 25305.2 Mbps | 45497.3 Mbps (79.79% improvement)
24 | 20104.1 Mbps | 37907.7 Mbps (88.55% improvement)
30 | 14702.4 Mbps | 30746.5 Mbps (109.12% improvement)
36 | 10801.5 Mbps | 26476.3 Mbps (145.11% improvement)

The results show drastic improvement for network intensive workloads.

[shakeel.butt@linux.dev: add BUILD_BUG_ON() for MEMCG_CHARGE_BATCH]
Link: https://lkml.kernel.org/r/rlsgeosg3j7v5nihhbxxxbv3xfy4ejvigihj7lkkbt3n6imyne@2apxx2jm2e57
[shakeel.butt@linux.dev: simplify refill_stock]
Link: https://lkml.kernel.org/r/as5cdsm4lraxupg3t6onep2ixql72za25hvd4x334dsoyo4apr@zyzl4vkuevuv
[hughd@google.com: it's better to stock nr_pages than the uninitialized stock_pages]
Link: https://lkml.kernel.org/r/d542d18f-1caa-6fea-e2c3-3555c87bcf64@google.com
[shakeel.butt@linux.dev: add comment per Michal and use DEFINE_PER_CPU_ALIGNED instead of DEFINE_PER_CPU per Vlastimil]
Link: https://lkml.kernel.org/r/dieeei3squ2gcnqxdjayvxbvzldr266rhnvtl3vjzsqevxkevf@ckui5vjzl2qg
Link: https://lkml.kernel.org/r/20250416180229.2902751-1-shakeel.butt@linux.dev
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Eric Dumaze <edumazet@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Soheil Hassas Yeganeh <soheil@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>