On 07/07/16 17:18, Sergey Fedorov wrote:
> On 05/07/16 19:18, Alex Bennée wrote:
>> 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.ben...@linaro.org>
>> Reviewed-by: Richard Henderson <r...@twiddle.net>
> Reviewed-by: Sergey Fedorov <sergey.fedo...@linaro.org>
Revoked:
(1) Acess to 'cpu->tb_flushed' must be safe (atomic_xchg() can help)
(2) A read from 'cpu->tb_jmp_cache' in tb_find_fast() can see a torn
write by memset() from tb_flush()
So I'm afraid, this series is dependent on safe tb_flush() implementation.
Kind regards,
Sergey
>
>> ---
>> v3 (base-patches)
>> - fix merge conflicts with Sergey's patch
>> v1 (hot path, split from base-patches series)
>> - revert name tweaking
>> - drop test jmp_list_next outside lock
>> - mention lock NOPs in comments
>> v2 (hot path)
>> - Add r-b tags
>> ---
>> cpu-exec.c | 48 +++++++++++++++++++++---------------------------
>> 1 file changed, 21 insertions(+), 27 deletions(-)
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index 10ce1cb..dd0bd50 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -291,35 +291,29 @@ 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) {
>>
>> -#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;
>> }
>> @@ -337,7 +331,6 @@ 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)) {
>> @@ -361,9 +354,10 @@ 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();
>> tb_add_jump(*last_tb, tb_exit, tb);
>> + tb_unlock();
>> }
>> - tb_unlock();
>> return tb;
>> }
>>
