Merge tag 'prlimit-tasklist_lock-for-v5.18' of...

Merge tag 'prlimit-tasklist_lock-for-v5.18' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace

Pull tasklist_lock optimizations from Eric Biederman:
 "prlimit and getpriority tasklist_lock optimizations

  The tasklist_lock popped up as a scalability bottleneck on some
  testing workloads. The readlocks in do_prlimit and set/getpriority are
  not necessary in all cases.

  Based on a cycles profile, it looked like ~87% of the time was spent
  in the kernel, ~42% of which was just trying to get *some* spinlock
  (queued_spin_lock_slowpath, not necessarily the tasklist_lock).

  The big offenders (with rough percentages in cycles of the overall
  trace):
   - do_wait 11%
   - setpriority 8% (done previously in commit 7f8ca0ed)
   - kill 8%
   - do_exit 5%
   - clone 3%
   - prlimit64 2%   (this patchset)
   - getrlimit 1%   (this patchset)

  I can't easily test this patchset on the original workload for various
  reasons. Instead, I used the microbenchmark below to at least verify
  there was some improvement. This patchset had a 28% speedup (12% from
  baseline to set/getprio, then another 14% for prlimit).

  This series used to do the setpriority case, but an almost identical
  change was merged as commit 7f8ca0ed ("kernel/sys.c: only take
  tasklist_lock for get/setpriority(PRIO_PGRP)") so that has been
  dropped from here.

  One interesting thing is that my libc's getrlimit() was calling
  prlimit64, so hoisting the read_lock(tasklist_lock) into sys_prlimit64
  had no effect - it essentially optimized the older syscalls only. I
  didn't do that in this patchset, but figured I'd mention it since it
  was an option from the previous patch's discussion"

micobenchmark.c:
---------------
	int main(int argc, char **argv)
	{
		pid_t child;
		struct rlimit rlim[1];

		fork(); fork(); fork(); fork(); fork(); fork();

		for (int i = 0; i < 5000; i++) {
			child = fork();
			if (child < 0)
				exit(1);
			if (child > 0) {
				usleep(1000);
				kill(child, SIGTERM);
				waitpid(child, NULL, 0);
			} else {
				for (;;) {
					setpriority(PRIO_PROCESS, 0,
						    getpriority(PRIO_PROCESS, 0));
					getrlimit(RLIMIT_CPU, rlim);
				}
			}
		}

		return 0;
	}

Link: https://lore.kernel.org/lkml/20211213220401.1039578-1-brho@google.com/ [v1]
Link: https://lore.kernel.org/lkml/20220105212828.197013-1-brho@google.com/ [v2]
Link: https://lore.kernel.org/lkml/20220106172041.522167-1-brho@google.com/ [v3]

* tag 'prlimit-tasklist_lock-for-v5.18' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
  prlimit: do not grab the tasklist_lock
  prlimit: make do_prlimit() static