From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42853) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bIGmY-0008VT-9W for qemu-devel@nongnu.org; Wed, 29 Jun 2016 10:48:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bIGmR-00047q-Cd for qemu-devel@nongnu.org; Wed, 29 Jun 2016 10:48:01 -0400 Received: from mail-wm0-x234.google.com ([2a00:1450:400c:c09::234]:36190) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bIGmQ-000473-KS for qemu-devel@nongnu.org; Wed, 29 Jun 2016 10:47:55 -0400 Received: by mail-wm0-x234.google.com with SMTP id f126so184746514wma.1 for ; Wed, 29 Jun 2016 07:47:54 -0700 (PDT) From: Alex =?utf-8?Q?Benn=C3=A9e?= In-reply-to: <5773DCCC.9080607@gmail.com> Date: Wed, 29 Jun 2016 15:47:58 +0100 Message-ID: <87r3bg1275.fsf@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Subject: Re: [Qemu-devel] [RFC v3 19/19] cpu-exec: remove tb_lock from the hot-path List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Sergey Fedorov Cc: mttcg@listserver.greensocs.com, qemu-devel@nongnu.org, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, cota@braap.org, bobby.prani@gmail.com, mark.burton@greensocs.com, pbonzini@redhat.com, jan.kiszka@siemens.com, rth@twiddle.net, peter.maydell@linaro.org, claudio.fontana@huawei.com, Peter Crosthwaite Sergey Fedorov 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 >> >> --- >> 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