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tcg: cpu-exec: remove tb_lock from the hot-path

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>

Message-Id: <20160715175852.30749-10-sergey.fedorov@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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stsquad authored and bonzini committed Jul 15, 2016
1 parent 2e1ae44 commit 518615c6503ad78d3bb67ddf1cd848c4a41de02e
Showing with 21 additions and 27 deletions.
  1. +21 −27 cpu-exec.c
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@@ -287,35 +287,29 @@ 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;
}
@@ -333,7 +327,6 @@ 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_rcu_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)) {
@@ -350,14 +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->invalid) {
tb_add_jump(last_tb, tb_exit, tb);
}
tb_unlock();
}
tb_unlock();
return tb;
}

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