[v2,3/6] 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>


---
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(-)

-- 
2.7.4

Sergey Fedorov <serge.fdrv@gmail.com> writes:

> 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.bennee@linaro.org>

>>> Reviewed-by: Richard Henderson <rth@twiddle.net>

>> Reviewed-by: Sergey Fedorov <sergey.fedorov@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()


But that is still the case already isn't it? While system emulation mode
is still single-threaded is it still safe?

>

> So I'm afraid, this series is dependent on safe tb_flush()

> implementation.


I've argue if it doesn't worsen the situation for either mode it's not
> dependant.



>

> 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;

>>>  }

>>>  



-- 
Alex Bennée