uprobes: Reuse return_instances between multiple uretprobes within task

Instead of constantly allocating and freeing very short-lived
struct return_instance, reuse it as much as possible within current
task. For that, store a linked list of reusable return_instances within
current->utask.

The only complication is that ri_timer() might be still processing such
return_instance. And so while the main uretprobe processing logic might
be already done with return_instance and would be OK to immediately
reuse it for the next uretprobe instance, it's not correct to
unconditionally reuse it just like that.

Instead we make sure that ri_timer() can't possibly be processing it by
using seqcount_t, with ri_timer() being "a writer", while
free_ret_instance() being "a reader". If, after we unlink return
instance from utask->return_instances list, we know that ri_timer()
hasn't gotten to processing utask->return_instances yet, then we can be
sure that immediate return_instance reuse is OK, and so we put it
onto utask->ri_pool for future (potentially, almost immediate) reuse.

This change shows improvements both in single CPU performance (by
avoiding relatively expensive kmalloc/free combon) and in terms of
multi-CPU scalability, where you can see that per-CPU throughput doesn't
decline as steeply with increased number of CPUs (which were previously
attributed to kmalloc()/free() through profiling):

	BASELINE (latest perf/core)
	===========================
	uretprobe-nop         ( 1 cpus):    1.898 ± 0.002M/s  (  1.898M/s/cpu)
	uretprobe-nop         ( 2 cpus):    3.574 ± 0.011M/s  (  1.787M/s/cpu)
	uretprobe-nop         ( 3 cpus):    5.279 ± 0.066M/s  (  1.760M/s/cpu)
	uretprobe-nop         ( 4 cpus):    6.824 ± 0.047M/s  (  1.706M/s/cpu)
	uretprobe-nop         ( 5 cpus):    8.339 ± 0.060M/s  (  1.668M/s/cpu)
	uretprobe-nop         ( 6 cpus):    9.812 ± 0.047M/s  (  1.635M/s/cpu)
	uretprobe-nop         ( 7 cpus):   11.030 ± 0.048M/s  (  1.576M/s/cpu)
	uretprobe-nop         ( 8 cpus):   12.453 ± 0.126M/s  (  1.557M/s/cpu)
	uretprobe-nop         (10 cpus):   14.838 ± 0.044M/s  (  1.484M/s/cpu)
	uretprobe-nop         (12 cpus):   17.092 ± 0.115M/s  (  1.424M/s/cpu)
	uretprobe-nop         (14 cpus):   19.576 ± 0.022M/s  (  1.398M/s/cpu)
	uretprobe-nop         (16 cpus):   22.264 ± 0.015M/s  (  1.391M/s/cpu)
	uretprobe-nop         (24 cpus):   33.534 ± 0.078M/s  (  1.397M/s/cpu)
	uretprobe-nop         (32 cpus):   43.262 ± 0.127M/s  (  1.352M/s/cpu)
	uretprobe-nop         (40 cpus):   53.252 ± 0.080M/s  (  1.331M/s/cpu)
	uretprobe-nop         (48 cpus):   55.778 ± 0.045M/s  (  1.162M/s/cpu)
	uretprobe-nop         (56 cpus):   56.850 ± 0.227M/s  (  1.015M/s/cpu)
	uretprobe-nop         (64 cpus):   62.005 ± 0.077M/s  (  0.969M/s/cpu)
	uretprobe-nop         (72 cpus):   66.445 ± 0.236M/s  (  0.923M/s/cpu)
	uretprobe-nop         (80 cpus):   68.353 ± 0.180M/s  (  0.854M/s/cpu)

	THIS PATCHSET (on top of latest perf/core)
	==========================================
	uretprobe-nop         ( 1 cpus):    2.253 ± 0.004M/s  (  2.253M/s/cpu)
	uretprobe-nop         ( 2 cpus):    4.281 ± 0.003M/s  (  2.140M/s/cpu)
	uretprobe-nop         ( 3 cpus):    6.389 ± 0.027M/s  (  2.130M/s/cpu)
	uretprobe-nop         ( 4 cpus):    8.328 ± 0.005M/s  (  2.082M/s/cpu)
	uretprobe-nop         ( 5 cpus):   10.353 ± 0.001M/s  (  2.071M/s/cpu)
	uretprobe-nop         ( 6 cpus):   12.513 ± 0.010M/s  (  2.086M/s/cpu)
	uretprobe-nop         ( 7 cpus):   14.525 ± 0.017M/s  (  2.075M/s/cpu)
	uretprobe-nop         ( 8 cpus):   15.633 ± 0.013M/s  (  1.954M/s/cpu)
	uretprobe-nop         (10 cpus):   19.532 ± 0.011M/s  (  1.953M/s/cpu)
	uretprobe-nop         (12 cpus):   21.405 ± 0.009M/s  (  1.784M/s/cpu)
	uretprobe-nop         (14 cpus):   24.857 ± 0.020M/s  (  1.776M/s/cpu)
	uretprobe-nop         (16 cpus):   26.466 ± 0.018M/s  (  1.654M/s/cpu)
	uretprobe-nop         (24 cpus):   40.513 ± 0.222M/s  (  1.688M/s/cpu)
	uretprobe-nop         (32 cpus):   54.180 ± 0.074M/s  (  1.693M/s/cpu)
	uretprobe-nop         (40 cpus):   66.100 ± 0.082M/s  (  1.652M/s/cpu)
	uretprobe-nop         (48 cpus):   70.544 ± 0.068M/s  (  1.470M/s/cpu)
	uretprobe-nop         (56 cpus):   74.494 ± 0.055M/s  (  1.330M/s/cpu)
	uretprobe-nop         (64 cpus):   79.317 ± 0.029M/s  (  1.239M/s/cpu)
	uretprobe-nop         (72 cpus):   84.875 ± 0.020M/s  (  1.179M/s/cpu)
	uretprobe-nop         (80 cpus):   92.318 ± 0.224M/s  (  1.154M/s/cpu)

For reference, with uprobe-nop we hit the following throughput:

	uprobe-nop            (80 cpus):  143.485 ± 0.035M/s  (  1.794M/s/cpu)

So now uretprobe stays a bit closer to that performance.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20241206002417.3295533-5-andrii@kernel.org

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