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authorPaolo Bonzini <pbonzini@redhat.com>2016-08-31 21:33:58 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2016-09-27 11:57:30 +0200
commitc265e976f4669fd65f5b47e6865f50d1cb66bd02 (patch)
treeb80f5c8500bd6ad92ee0474bce2b4186dd2540c5 /docs
parent3359baad36889b83df40b637ed993a4b816c4906 (diff)
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cpus-common: lock-free fast path for cpu_exec_start/end
Set cpu->running without taking the cpu_list lock, only requiring it if there is a concurrent exclusive section. This requires adding a new field to CPUState, which records whether a running CPU is being counted in pending_cpus. When an exclusive section is started concurrently with cpu_exec_start, cpu_exec_start can use the new field to determine if it has to wait for the end of the exclusive section. Likewise, cpu_exec_end can use it to see if start_exclusive is waiting for that CPU. This a separate patch for easier bisection of issues. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'docs')
-rw-r--r--docs/tcg-exclusive.promela53
1 files changed, 50 insertions, 3 deletions
diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
index feac679..c91cfca 100644
--- a/docs/tcg-exclusive.promela
+++ b/docs/tcg-exclusive.promela
@@ -13,7 +13,8 @@
* gcc pan.c -O2
* ./a.out -a
*
- * Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, TEST_EXPENSIVE.
+ * Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, USE_MUTEX,
+ * TEST_EXPENSIVE.
*/
// Define the missing parameters for the model
@@ -22,8 +23,10 @@
#warning defaulting to 2 CPU processes
#endif
-// the expensive test is not so expensive for <= 3 CPUs
-#if N_CPUS <= 3
+// the expensive test is not so expensive for <= 2 CPUs
+// If the mutex is used, it's also cheap (300 MB / 4 seconds) for 3 CPUs
+// For 3 CPUs and the lock-free option it needs 1.5 GB of RAM
+#if N_CPUS <= 2 || (N_CPUS <= 3 && defined USE_MUTEX)
#define TEST_EXPENSIVE
#endif
@@ -107,6 +110,8 @@ byte has_waiter[N_CPUS];
COND_BROADCAST(exclusive_resume); \
MUTEX_UNLOCK(mutex);
+#ifdef USE_MUTEX
+// Simple version using mutexes
#define cpu_exec_start(id) \
MUTEX_LOCK(mutex); \
exclusive_idle(); \
@@ -127,6 +132,48 @@ byte has_waiter[N_CPUS];
:: else -> skip; \
fi; \
MUTEX_UNLOCK(mutex);
+#else
+// Wait-free fast path, only needs mutex when concurrent with
+// an exclusive section
+#define cpu_exec_start(id) \
+ running[id] = 1; \
+ if \
+ :: pending_cpus -> { \
+ MUTEX_LOCK(mutex); \
+ if \
+ :: !has_waiter[id] -> { \
+ running[id] = 0; \
+ exclusive_idle(); \
+ running[id] = 1; \
+ } \
+ :: else -> skip; \
+ fi; \
+ MUTEX_UNLOCK(mutex); \
+ } \
+ :: else -> skip; \
+ fi;
+
+#define cpu_exec_end(id) \
+ running[id] = 0; \
+ if \
+ :: pending_cpus -> { \
+ MUTEX_LOCK(mutex); \
+ if \
+ :: has_waiter[id] -> { \
+ has_waiter[id] = 0; \
+ pending_cpus--; \
+ if \
+ :: pending_cpus == 1 -> COND_BROADCAST(exclusive_cond); \
+ :: else -> skip; \
+ fi; \
+ } \
+ :: else -> skip; \
+ fi; \
+ MUTEX_UNLOCK(mutex); \
+ } \
+ :: else -> skip; \
+ fi
+#endif
// Promela processes