[GIT PULL] prlimit and set/getpriority tasklist_lock optimizations

[Eric W. Biederman]

Linus,

Please pull the prlimit-tasklist_lock-for-v5.18 tag from the git tree:

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

  HEAD: 18c91bb2d87268d23868bf13508f5bc9cf04e89a prlimit: do not grab the tasklist_lock

From: Barret Rhoden <brho@xxxxxxxxxx>

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% (this patchset)
- 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).

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.

v3: https://lkml.kernel.org/r/20220106172041.522167-1-brho@xxxxxxxxxx
v2: https://lore.kernel.org/lkml/20220105212828.197013-1-brho@xxxxxxxxxx/
- update_rlimit_cpu on the group_leader instead of for_each_thread.
- update_rlimit_cpu still returns 0 or -ESRCH, even though we don't care
  about the error here.  it felt safer that way in case someone uses
  that function again.

v1: https://lore.kernel.org/lkml/20211213220401.1039578-1-brho@xxxxxxxxxx/

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;
}

Barret Rhoden (3):
  setpriority: only grab the tasklist_lock for PRIO_PGRP
  prlimit: make do_prlimit() static
  prlimit: do not grab the tasklist_lock

 include/linux/posix-timers.h   |   2 +-
 include/linux/resource.h       |   2 -
 kernel/sys.c                   | 127 +++++++++++++++++----------------
 kernel/time/posix-cpu-timers.c |  12 +++-
 4 files changed, 76 insertions(+), 67 deletions(-)

I have dropped the first change in this series as an almost identical
change was merged as commit 7f8ca0edfe07 ("kernel/sys.c: only take
tasklist_lock for get/setpriority(PRIO_PGRP)"). -- EWB

Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>