[Qemu-devel] [PATCH v4 09/12] tcg: cpu-exec: remove tb_lock from the hot-path

[Sergey Fedorov <sergey.fedorov@linaro.org>]

From: Alex Bennée <alex.bennee@linaro.org>

Lock contention in the hot path of moving between existing patched
TranslationBlocks is the main drag in multithreaded 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, atomically updated or designed to be used in
concurrent read situations (qht_lookup).

To keep things simple I removed the #ifdef CONFIG_USER_ONLY stuff as the
locks become NOPs anyway until the MTTCG work is completed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>

---
v2 (hot path)
 - Add r-b tags
v1 (hot path, split from base-patches series)
 - revert name tweaking
 - drop test jmp_list_next outside lock
 - mention lock NOPs in comments
v3 (base-patches)
 - fix merge conflicts with Sergey's patch
---
 cpu-exec.c | 48 +++++++++++++++++++++---------------------------
 1 file changed, 21 insertions(+), 27 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index e16df762f50a..bbaed5bb1978 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -286,35 +286,29 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
     TranslationBlock *tb;
 
     tb = tb_find_physical(cpu, pc, cs_base, flags);
-    if (tb) {
-        goto found;
-    }
+    if (!tb) {
 
-#ifdef CONFIG_USER_ONLY
-    /* 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;
-    }
-#endif
+        /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
+         * taken outside tb_lock. As system emulation is currently
+         * single threaded the locks are NOPs.
+         */
+        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);
+        }
 
-#ifdef CONFIG_USER_ONLY
-    mmap_unlock();
-#endif
+        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 */
     atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
     return tb;
 }
@@ -332,7 +326,6 @@ 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 || atomic_read(&tb->pc) != pc ||
                  atomic_read(&tb->cs_base) != cs_base ||
@@ -350,14 +343,15 @@ static inline TranslationBlock *tb_find_fast(CPUState *cpu,
 #endif
     /* See if we can patch the calling TB. */
     if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
+        tb_lock();
         /* Check if translation buffer has been flushed */
         if (cpu->tb_flushed) {
             cpu->tb_flushed = false;
         } else if (!tb_is_invalid(tb)) {
             tb_add_jump(last_tb, tb_exit, tb);
         }
+        tb_unlock();
     }
-    tb_unlock();
     return tb;
 }
 
-- 
2.9.1