kernel - Add debugging features

* Add lwkt.mplock_backtrace to force the kernel to print a backtrace
  when it encounters a mplock collision.  This is a bit problematic
  though as the backtrace can sometimes crash the kernel.

* Allow lwkt.mplock_yield_delay to be set to 0.

diff --git a/sys/kern/kern_mplock.c b/sys/kern/kern_mplock.c
index c280093..20cecc9 100644 (file)
--- a/sys/kern/kern_mplock.c
+++ b/sys/kern/kern_mplock.c
@@ -56,13 +56,16 @@
 
 #ifdef SMP
 static int chain_mplock = 0;
-static int bgl_yield = 10;
+static int mplock_yield = 10;
+static int mplock_backtrace;
 static __int64_t mplock_contention_count = 0;
 
 SYSCTL_INT(_lwkt, OID_AUTO, chain_mplock, CTLFLAG_RW, &chain_mplock, 0,
     "Chain IPI's to other CPU's potentially needing the MP lock when it is yielded");
-SYSCTL_INT(_lwkt, OID_AUTO, bgl_yield_delay, CTLFLAG_RW, &bgl_yield, 0,
+SYSCTL_INT(_lwkt, OID_AUTO, mplock_yield_delay, CTLFLAG_RW, &mplock_yield, 0,
     "Duration of delay when MP lock is temporarily yielded");
+SYSCTL_INT(_lwkt, OID_AUTO, mplock_backtrace, CTLFLAG_RW, &mplock_backtrace, 0,
+    "Output backplane when mplock contention occurs");
 SYSCTL_QUAD(_lwkt, OID_AUTO, mplock_contention_count, CTLFLAG_RW,
        &mplock_contention_count, 0, "spinning due to MPLOCK contention");
 
@@ -136,6 +139,10 @@ _get_mplock_contested(const char *file, int line)
        int nv;
        const void **stkframe = (const void **)&file;
 
+       if (mplock_backtrace > 0) {
+               --mplock_backtrace;
+               print_backtrace(-1);
+       }
        ++mplock_contention_count;
        for (;;) {
                ov = mp_lock;
@@ -240,11 +247,11 @@ yield_mplock(thread_t td)
 {
        int savecnt;
 
-       if (td->td_xpcount == 0) {
+       if (td->td_xpcount == 0 && mplock_yield > 0) {
                savecnt = td->td_mpcount;
                td->td_mpcount = 1;
                rel_mplock();
-               DELAY(bgl_yield);
+               DELAY(mplock_yield);
                get_mplock();
                td->td_mpcount = savecnt;
        }