linux-kernel - [PATCH v2] debug: More properly delay for secondary CPUs

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Date:   Fri, 21 Oct 2016 16:09:21 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Jason Wessel <jason.wessel@...driver.com>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        briannorris@...omium.org, Douglas Anderson <dianders@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: [PATCH v2] 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 debugger only to print their stack crawls shortly after the
kdb> prompt was written.

Elsewhere in kgdb we already use udelay(), so that should be safe enough
to use to implement our timeout.  We'll delay 1 ms for 1000 times, which
should give us a full second of delay (just like the old code wanted)
but allow us to notice that we're done every 1 ms.

Signed-off-by: Douglas Anderson <dianders@...omium.org>
---
Changes in v2:
- Use udelay, not __delay

 kernel/debug/debug_core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index 0874e2edd275..85a246feb442 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -61,6 +61,8 @@

 #include "debug_core.h"

+#define WAIT_CPUS_STOP_MS	1000
+
 static int kgdb_break_asap;

 struct debuggerinfo_struct kgdb_info[NR_CPUS];
@@ -598,11 +600,11 @@ 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 = WAIT_CPUS_STOP_MS;
 	while (kgdb_do_roundup && --time_left &&
 	       (atomic_read(&masters_in_kgdb) + atomic_read(&slaves_in_kgdb)) !=
 		   online_cpus)
-		cpu_relax();
+		udelay(1000);
 	if (!time_left)
 		pr_crit("Timed out waiting for secondary CPUs.\n");

-- 
2.8.0.rc3.226.g39d4020