[Xen-devel] [PATCH v6 19/20] xen/sched: add scheduling granularity enum

[Juergen Gross <jgross@xxxxxxxx>]

Add a scheduling granularity enum ("cpu", "core", "socket") for
specification of the scheduling granularity. Initially it is set to
"cpu", this can be modified by the new boot parameter (x86 only)
"sched-gran".

According to the selected granularity sched_granularity is set after
all cpus are online.

A test is added for all sched resources holding the same number of
cpus. Fall back to core- or cpu-scheduling in that case.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Dario Faggioli <dfaggioli@xxxxxxxx>
---
RFC V2:
- fixed freeing of sched_res when merging cpus
- rename parameter to "sched-gran" (Jan Beulich)
- rename parameter option from "thread" to "cpu" (Jan Beulich)

V1:
- rename scheduler_smp_init() to scheduler_gran_init(), let it be called
  by cpupool_init()
- avoid using literal cpu number 0 in scheduler_percpu_init() (Jan Beulich)
- style correction (Jan Beulich)
- fallback to smaller granularity instead of panic in case of
  unbalanced cpu configuration

V2:
- style changes (Jan Beulich)
- introduce CONFIG_HAS_SCHED_GRANULARITY (Jan Beulich)

V4:
- move code to cpupool.c
---
 xen/arch/x86/Kconfig |  1 +
 xen/common/Kconfig   |  3 ++
 xen/common/cpupool.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 84 insertions(+)

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 288dc6c042..3f88adae97 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -22,6 +22,7 @@ config X86
        select HAS_PASSTHROUGH
        select HAS_PCI
        select HAS_PDX
+       select HAS_SCHED_GRANULARITY
        select HAS_UBSAN
        select HAS_VPCI if !PV_SHIM_EXCLUSIVE && HVM
        select NEEDS_LIBELF
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 16829f6274..e9247871a8 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -63,6 +63,9 @@ config HAS_GDBSX
 config HAS_IOPORTS
        bool
 
+config HAS_SCHED_GRANULARITY
+       bool
+
 config NEEDS_LIBELF
        bool
 
diff --git a/xen/common/cpupool.c b/xen/common/cpupool.c
index f7a13c7a4c..4d3adbdd8d 100644
--- a/xen/common/cpupool.c
+++ b/xen/common/cpupool.c
@@ -17,6 +17,7 @@
 #include <xen/percpu.h>
 #include <xen/sched.h>
 #include <xen/sched-if.h>
+#include <xen/warning.h>
 #include <xen/keyhandler.h>
 #include <xen/cpu.h>
 
@@ -37,6 +38,83 @@ static DEFINE_SPINLOCK(cpupool_lock);
 static enum sched_gran __read_mostly opt_sched_granularity = SCHED_GRAN_cpu;
 static unsigned int __read_mostly sched_granularity = 1;
 
+#ifdef CONFIG_HAS_SCHED_GRANULARITY
+static int __init sched_select_granularity(const char *str)
+{
+    if ( strcmp("cpu", str) == 0 )
+        opt_sched_granularity = SCHED_GRAN_cpu;
+    else if ( strcmp("core", str) == 0 )
+        opt_sched_granularity = SCHED_GRAN_core;
+    else if ( strcmp("socket", str) == 0 )
+        opt_sched_granularity = SCHED_GRAN_socket;
+    else
+        return -EINVAL;
+
+    return 0;
+}
+custom_param("sched-gran", sched_select_granularity);
+#endif
+
+static unsigned int __init cpupool_check_granularity(void)
+{
+    unsigned int cpu;
+    unsigned int siblings, gran = 0;
+
+    if ( opt_sched_granularity == SCHED_GRAN_cpu )
+        return 1;
+
+    for_each_online_cpu ( cpu )
+    {
+        siblings = cpumask_weight(sched_get_opt_cpumask(opt_sched_granularity,
+                                                        cpu));
+        if ( gran == 0 )
+            gran = siblings;
+        else if ( gran != siblings )
+            return 0;
+    }
+
+    sched_disable_smt_switching = true;
+
+    return gran;
+}
+
+/* Setup data for selected scheduler granularity. */
+static void __init cpupool_gran_init(void)
+{
+    unsigned int gran = 0;
+    const char *fallback = NULL;
+
+    while ( gran == 0 )
+    {
+        gran = cpupool_check_granularity();
+
+        if ( gran == 0 )
+        {
+            switch ( opt_sched_granularity )
+            {
+            case SCHED_GRAN_core:
+                opt_sched_granularity = SCHED_GRAN_cpu;
+                fallback = "Asymmetric cpu configuration.\n"
+                           "Falling back to sched-gran=cpu.\n";
+                break;
+            case SCHED_GRAN_socket:
+                opt_sched_granularity = SCHED_GRAN_core;
+                fallback = "Asymmetric cpu configuration.\n"
+                           "Falling back to sched-gran=core.\n";
+                break;
+            default:
+                ASSERT_UNREACHABLE();
+                break;
+            }
+        }
+    }
+
+    if ( fallback )
+        warning_add(fallback);
+
+    sched_granularity = gran;
+}
+
 unsigned int cpupool_get_granularity(const struct cpupool *c)
 {
     return c ? sched_granularity : 1;
@@ -871,6 +949,8 @@ static int __init cpupool_init(void)
     unsigned int cpu;
     int err;
 
+    cpupool_gran_init();
+
     cpupool0 = cpupool_create(0, 0, &err);
     BUG_ON(cpupool0 == NULL);
     cpupool_put(cpupool0);
-- 
2.16.4


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