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Re: [Qemu-devel] [RFC v3 19/19] cpu-exec: remove tb_lock from the hot-pa
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [RFC v3 19/19] cpu-exec: remove tb_lock from the hot-path |
Date: |
Wed, 29 Jun 2016 15:47:58 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.0.95.6 |
Sergey Fedorov <address@hidden> writes:
> On 03/06/16 23:40, Alex Bennée wrote:
>> 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 <address@hidden>
>>
>> ---
>> 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;
>
> Hmm, seems like I forgot to convert this into atomic_set() in the
> previous patch...
OK, can you fix that in your quick fixes series?
>
>> return tb;
>> }
>>
>> static inline TranslationBlock *tb_find_fast(CPUState *cpu,
>> - TranslationBlock **last_tb,
>> + TranslationBlock **ltbp,
>
> I'm not sure if it is more readable...
I'll revert. I was trying to keep line lengths short :-/
>
>> 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]) {
>
> If we're going to check this outside of tb_lock we have to do this with
> atomic_{read,set}(). However, I think it is rare case to race on
> tb_add_jump() so probably it is okay to do the check under tb_lock.
It's checking for NULL, it gets re-checked in tb_add_jump while under
lock so I the setting race should be OK I think.
>
>> + tb_lock();
>> + tb_add_jump(last_tb, tb_exit, tb);
>> + tb_unlock();
>> }
>> - tb_unlock();
>> return tb;
>> }
>>
>
> Kind regards,
> Sergey
--
Alex Bennée
- [Qemu-devel] [RFC v3 09/19] target-arm/arm-powerctl: wake up sleeping CPUs, (continued)
- [Qemu-devel] [RFC v3 09/19] target-arm/arm-powerctl: wake up sleeping CPUs, Alex Bennée, 2016/06/03
- [Qemu-devel] [RFC v3 10/19] tcg: cpus rm tcg_exec_all(), Alex Bennée, 2016/06/03
- [Qemu-devel] [RFC v3 18/19] tcg: Ensure safe TB lookup out of 'tb_lock', Alex Bennée, 2016/06/03
- [Qemu-devel] [RFC v3 17/19] tcg: enable thread-per-vCPU, Alex Bennée, 2016/06/03
- [Qemu-devel] [RFC v3 15/19] tcg: drop global lock during TCG code execution, Alex Bennée, 2016/06/03
- [Qemu-devel] [RFC v3 19/19] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/06/03
[Qemu-devel] [RFC v3 16/19] tcg: move locking for tb_invalidate_phys_page_range up, Alex Bennée, 2016/06/03
Re: [Qemu-devel] [RFC v3 00/19] Base enabling patches for MTTCG, Pranith Kumar, 2016/06/04