ipvs: use kthreads for stats estimation

Estimating all entries in single list in timer context
by single CPU causes large latency with multiple IPVS rules
as reported in [1], [2], [3].

Spread the estimator structures in multiple chains and
use kthread(s) for the estimation. The chains are processed
in multiple (50) timer ticks to ensure the 2-second interval
between estimations with some accuracy. Every chain is
processed under RCU lock.

Every kthread works over its own data structure and all
such contexts are attached to array. The contexts can be
preserved while the kthread tasks are stopped or restarted.
When estimators are removed, unused kthread contexts are
released and the slots in array are left empty.

First kthread determines parameters to use, eg. maximum
number of estimators to process per kthread based on
chain's length (chain_max), allowing sub-100us cond_resched
rate and estimation taking up to 1/8 of the CPU capacity
to avoid any problems if chain_max is not correctly
calculated.

chain_max is calculated taking into account factors
such as CPU speed and memory/cache speed where the
cache_factor (4) is selected from real tests with
current generation of CPU/NUMA configurations to
correct the difference in CPU usage between
cached (during calc phase) and non-cached (working) state
of the estimated per-cpu data.

First kthread also plays the role of distributor of
added estimators to all kthreads, keeping low the
time to add estimators. The optimization is based on
the fact that newly added estimator should be estimated
after 2 seconds, so we have the time to offload the
adding to chain from controlling process to kthread 0.

The allocated kthread context may grow from 1 to 50
allocated structures for timer ticks which saves memory for
setups with small number of estimators.

We also add delayed work est_reload_work that will
make sure the kthread tasks are properly started/stopped.

ip_vs_start_estimator() is changed to report errors
which allows to safely store the estimators in
allocated structures.

Many thanks to Jiri Wiesner for his valuable comments
and for spending a lot of time reviewing and testing
the changes on different platforms with 48-256 CPUs and
1-8 NUMA nodes under different cpufreq governors.

[1] Report from Yunhong Jiang:
https://lore.kernel.org/netdev/D25792C1-1B89-45DE-9F10-EC350DC04ADC@gmail.com/
[2]
https://marc.info/?l=linux-virtual-server&m=159679809118027&w=2
[3] Report from Dust:
https://archive.linuxvirtualserver.org/html/lvs-devel/2020-12/msg00000.html

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Cc: yunhong-cgl jiang <xintian1976@gmail.com>
Cc: "dust.li" <dust.li@linux.alibaba.com>
Reviewed-by: Jiri Wiesner <jwiesner@suse.de>
Tested-by: Jiri Wiesner <jwiesner@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

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