Lock contention in the hot path of moving between existing patched
TranslationBlocks is the main drag on MTTCG performance. This patch
pushes the tb_lock() usage down to the two places that really need it:
- code generation (tb_gen_code)
- jump patching (tb_add_jump)
The rest of the code doesn't really need to hold a lock as it is either
using per-CPU structures or designed to be used in concurrent read
situations (qht_lookup).
Signed-off-by: Alex Bennée <alex.ben...@linaro.org>
---
v3
- fix merge conflicts with Sergey's patch
---
cpu-exec.c | 59 ++++++++++++++++++++++++++++++-----------------------------
1 file changed, 30 insertions(+), 29 deletions(-)
diff --git a/cpu-exec.c b/cpu-exec.c
index b017643..4af0b52 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -298,41 +298,38 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
* Pairs with smp_wmb() in tb_phys_invalidate(). */
smp_rmb();
tb = tb_find_physical(cpu, pc, cs_base, flags);
- if (tb) {
- goto found;
- }
+ if (!tb) {
- /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
- * taken outside tb_lock. Since we're momentarily dropping
- * tb_lock, there's a chance that our desired tb has been
- * translated.
- */
- tb_unlock();
- mmap_lock();
- tb_lock();
- tb = tb_find_physical(cpu, pc, cs_base, flags);
- if (tb) {
- mmap_unlock();
- goto found;
- }
+ /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
+ * taken outside tb_lock.
+ */
+ mmap_lock();
+ tb_lock();
- /* if no translated code available, then translate it now */
- tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
+ /* There's a chance that our desired tb has been translated while
+ * taking the locks so we check again inside the lock.
+ */
+ tb = tb_find_physical(cpu, pc, cs_base, flags);
+ if (!tb) {
+ /* if no translated code available, then translate it now */
+ tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
+ }
- mmap_unlock();
+ tb_unlock();
+ mmap_unlock();
+ }
-found:
- /* we add the TB in the virtual pc hash table */
+ /* We add the TB in the virtual pc hash table for the fast lookup */
cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
return tb;
}
static inline TranslationBlock *tb_find_fast(CPUState *cpu,
- TranslationBlock **last_tb,
+ TranslationBlock **ltbp,
int tb_exit)
{
CPUArchState *env = (CPUArchState *)cpu->env_ptr;
- TranslationBlock *tb;
+ TranslationBlock *tb, *last_tb;
target_ulong cs_base, pc;
uint32_t flags;
@@ -340,7 +337,6 @@ static inline TranslationBlock *tb_find_fast(CPUState *cpu,
always be the same before a given translated block
is executed. */
cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
- tb_lock();
tb = atomic_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
tb->flags != flags)) {
@@ -350,7 +346,7 @@ static inline TranslationBlock *tb_find_fast(CPUState *cpu,
/* Ensure that no TB jump will be modified as the
* translation buffer has been flushed.
*/
- *last_tb = NULL;
+ *ltbp = NULL;
cpu->tb_flushed = false;
}
#ifndef CONFIG_USER_ONLY
@@ -359,14 +355,19 @@ static inline TranslationBlock *tb_find_fast(CPUState
*cpu,
* spanning two pages because the mapping for the second page can change.
*/
if (tb->page_addr[1] != -1) {
- *last_tb = NULL;
+ *ltbp = NULL;
}
#endif
+
/* See if we can patch the calling TB. */
- if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
- tb_add_jump(*last_tb, tb_exit, tb);
+ last_tb = *ltbp;
+ if (!qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN) &&
+ last_tb &&
+ !last_tb->jmp_list_next[tb_exit]) {
+ tb_lock();
+ tb_add_jump(last_tb, tb_exit, tb);
+ tb_unlock();
}
- tb_unlock();
return tb;
}
--
2.7.4
