svn commit: r334026 - head/sys/kern

[Mateusz Guzik]

Author: mjg
Date: Tue May 22 08:27:33 2018
New Revision: 334026
URL: https://svnweb.freebsd.org/changeset/base/334026

Log:
  Reduce sdt-related branch-fest in mi_switch.
  
  The code was evaluating flags before resorting to checking if dtrace is
  enabled. This was inducing forward jumps in the common case.

Modified:
  head/sys/kern/kern_synch.c

Modified: head/sys/kern/kern_synch.c
==============================================================================
--- head/sys/kern/kern_synch.c	Tue May 22 07:56:58 2018	(r334025)
+++ head/sys/kern/kern_synch.c	Tue May 22 08:27:33 2018	(r334026)
@@ -431,8 +431,9 @@ mi_switch(int flags, struct thread *newtd)
 	CTR4(KTR_PROC, "mi_switch: old thread %ld (td_sched %p, pid %ld, %s)",
 	    td->td_tid, td_get_sched(td), td->td_proc->p_pid, td->td_name);
 #ifdef KDTRACE_HOOKS
-	if ((flags & SW_PREEMPT) != 0 || ((flags & SW_INVOL) != 0 &&
-	    (flags & SW_TYPE_MASK) == SWT_NEEDRESCHED))
+	if (__predict_false(sdt_probes_enabled) &&
+	    ((flags & SW_PREEMPT) != 0 || ((flags & SW_INVOL) != 0 &&
+	    (flags & SW_TYPE_MASK) == SWT_NEEDRESCHED)))
 		SDT_PROBE0(sched, , , preempt);
 #endif
 	sched_switch(td, newtd, flags);