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Subject[PATCH] hyperthreading scheduler improvement
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Linus,

This patch implements a per-arch load balancing scheme for P4s to better
balance across the virtual CPUs (e.g. prefer fully unused CPUs to a
single available HT unit). This is the same logic in 2.4-ac, 2.4-aa,
etc.

This patch uses the previously posted per-arch load balancing
infrastructure.

The new logic is keyed off of a new CONFIG_X86_HYPERTHREADING value
which is currently set off of CONFIG_MPENTIUM4. Thus, only kernels
compiled for P4s will receive the new logic. It is safe to make the
code available to all x86s, however.

As I am not fortunate enough to have dual Xeons at my disposal, this
patch is largely untested for performance, although it is present in -aa
and -ac. I suspect this could show very acceptable performance gains.

Robert Love

diff -urN linux-2.5.31/arch/i386/config.in linux/arch/i386/config.in
--- linux-2.5.31/arch/i386/config.in Sat Aug 10 21:41:25 2002
+++ linux/arch/i386/config.in Sat Aug 24 22:07:10 2002
@@ -103,6 +103,7 @@
define_bool CONFIG_X86_TSC y
define_bool CONFIG_X86_GOOD_APIC y
define_bool CONFIG_X86_USE_PPRO_CHECKSUM y
+ define_bool CONFIG_X86_HYPERTHREADING y
fi
if [ "$CONFIG_MK6" = "y" ]; then
define_int CONFIG_X86_L1_CACHE_SHIFT 5
diff -urN linux-2.5.31/include/asm-i386/processor.h linux/include/asm-i386/processor.h
--- linux-2.5.31/include/asm-i386/processor.h Sat Aug 10 21:41:18 2002
+++ linux/include/asm-i386/processor.h Sat Aug 24 22:07:10 2002
@@ -488,4 +488,6 @@

#endif

+#define ARCH_HAS_SMP_BALANCE
+
#endif /* __ASM_I386_PROCESSOR_H */
diff -urN linux-2.5.31/include/asm-i386/smp_balance.h linux/include/asm-i386/smp_balance.h
--- linux-2.5.31/include/asm-i386/smp_balance.h Wed Dec 31 19:00:00 1969
+++ linux/include/asm-i386/smp_balance.h Sat Aug 24 22:26:15 2002
@@ -0,0 +1,55 @@
+#ifndef _ASM_SMP_BALANCE_H
+#define _ASM_SMP_BALANCE_H
+
+/*
+ * We have an architecture-specific SMP load balancer to improve
+ * scheduling behavior on hyperthreaded CPUs. Since only P4s have
+ * HT, we only use the code if CONFIG_MPENTIUM4 is set.
+ *
+ * Distributions may want to make this unconditional to support all
+ * x86 machines on one kernel. The overhead in the non-P4 case is
+ * minimal while the benefit to SMP P4s is probably decent.
+ */
+#if defined(CONFIG_X86_HYPERTHREADING)
+
+/*
+ * Find any idle processor package (i.e. both virtual processors are idle)
+ */
+static inline int find_idle_package(int this_cpu)
+{
+ int i;
+
+ i = this_cpu;
+
+ for (i = (this_cpu + 1) % NR_CPUS;
+ i != this_cpu;
+ i = (i + 1) % NR_CPUS) {
+ int sibling = cpu_sibling_map[i];
+
+ if (idle_cpu(i) && idle_cpu(sibling))
+ return i;
+ }
+ return -1; /* not found */
+}
+
+static inline int arch_load_balance(int this_cpu, int idle)
+{
+ /* Special hack for hyperthreading */
+ if (unlikely(smp_num_siblings > 1 && idle && !idle_cpu(cpu_sibling_map[this_cpu]))) {
+ int found;
+ struct runqueue *rq_target;
+
+ if ((found = find_idle_package(this_cpu)) >= 0 ) {
+ rq_target = cpu_rq(found);
+ resched_task(rq_target->idle);
+ return 1;
+ }
+ }
+ return 0;
+}
+
+#else
+#define arch_load_balance(x, y) (0)
+#endif
+
+#endif /* _ASM_SMP_BALANCE_H */


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