[PATCH] x86/boot: Improve the boot watchdog determination of stuck cpus

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Right now, check_nmi_watchdog() has two processing loops over all online CPUs
using prev_nmi_count as storage.

Use a cpumask_t instead (1/32th as much initdata) and have wait_for_nmis()
make the determination of whether it is stuck, rather than having both
functions needing to agree on how many ticks mean stuck.

More importantly though, it means we can use the standard cpumask
infrastructure, including turning this:

  (XEN) Brought up 512 CPUs
  (XEN) Testing NMI watchdog on all CPUs: 
{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278,279,280,281,282,283,284,285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303,304,305,306,307,308,309,310,311,312,313,314,315,316,317,318,319,320,321,322,323,324,325,326,327,328,329,330,331,332,333,334,335,336,337,338,339,340,341,342,343,344,345,346,347,348,349,350,351,352,353,354,355,356,357,358,359,360,361,362,363,364,365,366,367,368,369,370,371,372,373,374,375,376,377,378,379,380,381,382,383,384,385,386,387,388,389,390,391,392,393,394,395,396,397,398,399,400,401,402,403,404,405,406,407,408,409,410,411,412,413,414,415,416,417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,441,442,443,444,445,446,447,448,449,450,451,452,453,454,455,456,457,458,459,460,461,462,463,464,465,466,467,468,469,470,471,472,473,474,475,476,477,478,479,480,481,482,483,484,485,486,487,488,489,490,491,492,493,494,495,496,497,498,499,500,501,502,503,504,505,506,507,508,509,510,511}
 stuck

into the rather more manageable:

  (XEN) Brought up 512 CPUs
  (XEN) Testing NMI watchdog on all CPUs: {0-511} stuck

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
---
 xen/arch/x86/nmi.c | 35 ++++++++++++++---------------------
 1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/xen/arch/x86/nmi.c b/xen/arch/x86/nmi.c
index 633dc59bf830..8994c50cb5e4 100644
--- a/xen/arch/x86/nmi.c
+++ b/xen/arch/x86/nmi.c
@@ -151,6 +151,8 @@ static int nmi_active;
 
 static void __init cf_check wait_for_nmis(void *p)
 {
+    cpumask_t *stuck_cpus = p;
+    unsigned int cpu = smp_processor_id();
     unsigned int start_count = this_cpu(nmi_count);
     unsigned long ticks = 10 * 1000 * cpu_khz / nmi_hz;
     unsigned long s, e;
@@ -159,44 +161,35 @@ static void __init cf_check wait_for_nmis(void *p)
     do {
         cpu_relax();
         if ( this_cpu(nmi_count) >= start_count + 2 )
-            break;
+            return;
+
         e = rdtsc();
-    } while( e - s < ticks );
+    } while ( e - s < ticks );
+
+    /* Timeout.  Mark ourselves as stuck. */
+    cpumask_set_cpu(cpu, stuck_cpus);
 }
 
 void __init check_nmi_watchdog(void)
 {
-    static unsigned int __initdata prev_nmi_count[NR_CPUS];
-    unsigned int cpu;
-    char sep = '{';
-    bool ok = true;
+    static cpumask_t __initdata stuck_cpus;
 
     if ( nmi_watchdog == NMI_NONE )
         return;
 
     printk("Testing NMI watchdog on all CPUs: ");
 
-    for_each_online_cpu ( cpu )
-        prev_nmi_count[cpu] = per_cpu(nmi_count, cpu);
-
     /*
      * Wait at most 10 ticks for 2 watchdog NMIs on each CPU.
      * Busy-wait on all CPUs: the LAPIC counter that the NMI watchdog
      * uses only runs while the core's not halted
      */
-    on_selected_cpus(&cpu_online_map, wait_for_nmis, NULL, 1);
+    on_selected_cpus(&cpu_online_map, wait_for_nmis, &stuck_cpus, 1);
 
-    for_each_online_cpu ( cpu )
-    {
-        if ( per_cpu(nmi_count, cpu) - prev_nmi_count[cpu] < 2 )
-        {
-            printk("%c%u", sep, cpu);
-            sep = ',';
-            ok = false;
-        }
-    }
-
-    printk("%s\n", ok ? "ok" : "} stuck");
+    if ( cpumask_empty(&stuck_cpus) )
+        printk("ok\n");
+    else
+        printk("{%*pbl} stuck\n", CPUMASK_PR(&stuck_cpus));
 
     /*
      * Now that we know it works we can reduce NMI frequency to

base-commit: d638e304f13a5ef7d125de5ace5f7828a7b25bac
-- 
2.30.2