audit: Reduce overhead using a coarse clock

Commit 2115bb25 ("audit: Use timespec64 to represent audit timestamps")
noted that audit timestamps were not y2038 safe and used a 64-bit
timestamp. In itself, this makes sense but the conversion was from
CURRENT_TIME to ktime_get_real_ts64() which is a heavier call to record
an accurate timestamp which is required in some, but not all, cases. The
impact is that when auditd is running without any rules that all syscalls
have higher overhead. This is visible in the sysbench-thread benchmark as
a 11.5% performance hit. That benchmark is dumb as rocks but it's also
visible in redis as an 8-10% hit on all operations which is of greater
concern. It is somewhat stupid of audit to track syscalls without any
rules related to syscalls but that is how it behaves.

The overhead can be directly measured with perf comparing 4.9 with 4.12

4.9
     7.76%  sysbench         [kernel.vmlinux]    [k] __schedule
     7.62%  sysbench         [kernel.vmlinux]    [k] _raw_spin_lock
     7.37%  sysbench         libpthread-2.22.so  [.] __lll_lock_elision
     7.29%  sysbench         [kernel.vmlinux]    [.] syscall_return_via_sysret
     6.59%  sysbench         [kernel.vmlinux]    [k] native_sched_clock
     5.21%  sysbench         libc-2.22.so        [.] __sched_yield
     4.38%  sysbench         [kernel.vmlinux]    [k] entry_SYSCALL_64
     4.28%  sysbench         [kernel.vmlinux]    [k] do_syscall_64
     3.49%  sysbench         libpthread-2.22.so  [.] __lll_unlock_elision
     3.13%  sysbench         [kernel.vmlinux]    [k] __audit_syscall_exit
     2.87%  sysbench         [kernel.vmlinux]    [k] update_curr
     2.73%  sysbench         [kernel.vmlinux]    [k] pick_next_task_fair
     2.31%  sysbench         [kernel.vmlinux]    [k] syscall_trace_enter
     2.20%  sysbench         [kernel.vmlinux]    [k] __audit_syscall_entry
.....
     0.00%  swapper          [kernel.vmlinux]    [k] read_tsc

4.12
     7.84%  sysbench         [kernel.vmlinux]    [k] __schedule
     7.05%  sysbench         [kernel.vmlinux]    [k] _raw_spin_lock
     6.57%  sysbench         libpthread-2.22.so  [.] __lll_lock_elision
     6.50%  sysbench         [kernel.vmlinux]    [.] syscall_return_via_sysret
     5.95%  sysbench         [kernel.vmlinux]    [k] read_tsc
     5.71%  sysbench         [kernel.vmlinux]    [k] native_sched_clock
     4.78%  sysbench         libc-2.22.so        [.] __sched_yield
     4.30%  sysbench         [kernel.vmlinux]    [k] entry_SYSCALL_64
     3.94%  sysbench         [kernel.vmlinux]    [k] do_syscall_64
     3.37%  sysbench         libpthread-2.22.so  [.] __lll_unlock_elision
     3.32%  sysbench         [kernel.vmlinux]    [k] __audit_syscall_exit
     2.91%  sysbench         [kernel.vmlinux]    [k] __getnstimeofday64

Note the additional overhead from read_tsc which goes from 0% to 5.95%.
This is on a single-socket E3-1230 but similar overheads have been measured
on an older machine which the patch also eliminates.

The patch in question has no explanation as to why a fully-accurate timestamp
is required and is likely an oversight.  Using a coarser, but monotically
increasing, timestamp the overhead can be eliminated.  While it can be
worked around by configuring or disabling audit, it's tricky enough to
detect that a kernel fix is justified. With this patch, we see the following;

sysbenchthread
                              4.9.0                 4.12.0                 4.12.0
                            vanilla                vanilla            coarse-v1r1
Amean     1         1.49 (   0.00%)        1.66 ( -11.42%)        1.51 (  -1.34%)
Amean     3         1.48 (   0.00%)        1.65 ( -11.45%)        1.50 (  -0.96%)
Amean     5         1.49 (   0.00%)        1.67 ( -12.31%)        1.51 (  -1.83%)
Amean     7         1.49 (   0.00%)        1.66 ( -11.72%)        1.50 (  -0.67%)
Amean     12        1.48 (   0.00%)        1.65 ( -11.57%)        1.52 (  -2.89%)
Amean     16        1.49 (   0.00%)        1.65 ( -11.13%)        1.51 (  -1.73%)

The benchmark is reporting the time required for different thread counts to
lock/unlock a private mutex which, while dense, demonstrates the syscall
overhead. This is showing that 4.12 took a 11-12% hit but the overhead is
almost eliminated by the patch. While the variance is not reported here,
it's well within the noise with the patch applied.
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit.c

@@ -1662,7 +1662,7 @@ static inline void audit_get_stamp(struct audit_context *ctx,
 				   struct timespec64 *t, unsigned int *serial)
 {
 	if (!ctx || !auditsc_get_stamp(ctx, t, serial)) {
-		ktime_get_real_ts64(t);
+		*t = current_kernel_time64();
 		*serial = audit_serial();
 	}
 }

kernel/auditsc.c

@@ -1536,7 +1536,7 @@ void __audit_syscall_entry(int major, unsigned long a1, unsigned long a2,
 		return;
 
 	context->serial     = 0;
-	ktime_get_real_ts64(&context->ctime);
+	context->ctime = current_kernel_time64();
 	context->in_syscall = 1;
 	context->current_state  = state;
 	context->ppid       = 0;