debug: More properly delay for secondary CPUs

We've got a delay loop waiting for secondary CPUs. That loop uses loops_per_jiffy. However, loops_per_jiffy doesn't actually mean how many tight loops make up a jiffy on all architectures. It is quite common to see things like this in the boot log:
Calibrating delay loop (skipped), value calculated using timer frequency.. 48.00 BogoMIPS (lpj=24000)

In my case I was seeing lots of cases where other CPUs timed out entering the debugging only to print their stack crawls shortly after the kdb> prompt was written.

It appears that other code with similar loops (like __spin_lock_debug) adds an extra __delay(1) in there which makes it work better. Presumably the __delay(1) is very safe. At least on modern ARM/ARM64 systems it will just do a CP15 instruction, which should be safe. On older ARM systems it will fall back to an actual delay loop, or perhaps another memory-mapped timer. On other platforms it must be safe too or it wouldn't be used in __spin_lock_debug.

Note that we use __delay(100) instead of __delay(1) so we can get a little closer to a more accurate timeout on systems where __delay() is backed by a timer. It's better to have a more accurate timeout and the only penalty is that we might wait an extra 99 "loops" before we enter the debugger.

Signed-off-by Douglas Anderson <[email protected]>

---
 kernel/debug/debug_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

kernel/debug/debug_core.c +2 -2

index 0874e2edd275..454150d98dbc 100644
@@ -598,11 +598,11 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 	/*
 	 * Wait for the other CPUs to be notified and be waiting for us:
 	 */
-	time_left = loops_per_jiffy * HZ;
+	time_left = DIV_ROUND_UP(loops_per_jiffy * HZ, 100);
 	while (kgdb_do_roundup && --time_left &&
 	       (atomic_read(&masters_in_kgdb) + atomic_read(&slaves_in_kgdb)) !=
 		   online_cpus)
-		cpu_relax();
+		__delay(100);
 	if (!time_left)
 		pr_crit("Timed out waiting for secondary CPUs.\n");
 

We've got a delay loop waiting for secondary CPUs. That loop uses loops_per_jiffy. However, loops_per_jiffy doesn't actually mean how many tight loops make up a jiffy on all architectures. It is quite common to see things like this in the boot log:
Calibrating delay loop (skipped), value calculated using timer frequency.. 48.00 BogoMIPS (lpj=24000)

In my case I was seeing lots of cases where other CPUs timed out entering the debugging only to print their stack crawls shortly after the kdb> prompt was written.

It appears that other code with similar loops (like __spin_lock_debug) adds an extra __delay(1) in there which makes it work better. Presumably the __delay(1) is very safe. At least on modern ARM/ARM64 systems it will just do a CP15 instruction, which should be safe. On older ARM systems it will fall back to an actual delay loop, or perhaps another memory-mapped timer. On other platforms it must be safe too or it wouldn't be used in __spin_lock_debug.

Note that we use __delay(100) instead of __delay(1) so we can get a little closer to a more accurate timeout on systems where __delay() is backed by a timer. It's better to have a more accurate timeout and the only penalty is that we might wait an extra 99 "loops" before we enter the debugger.

kernel/debug/debug_core.c +0 -0

@@ -598,11 +598,11 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 	/*
 	 * Wait for the other CPUs to be notified and be waiting for us:
 	 */