Messages in this thread Patch in this message |  | | From | Douglas Anderson <> | Subject | [PATCH] debug: More properly delay for secondary CPUs | Date | Fri, 14 Oct 2016 11:41:21 -0700 |
| |
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 <dianders@chromium.org> --- kernel/debug/debug_core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c index 0874e2edd275..454150d98dbc 100644 --- a/kernel/debug/debug_core.c +++ b/kernel/debug/debug_core.c @@ -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"); -- 2.8.0.rc3.226.g39d4020
|  |