[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path |
Date: |
Tue, 05 Jul 2016 12:14:22 +0100 |
User-agent: |
mu4e 0.9.17; emacs 25.0.95.7 |
Emilio G. Cota <address@hidden> writes:
> On Mon, Jul 04, 2016 at 12:45:52 +0100, Alex Bennée wrote:
>>
>> Emilio G. Cota <address@hidden> writes:
>>
>> > On Fri, Jul 01, 2016 at 17:16:10 +0100, 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.
>> >
>> > From a scalability point of view it would be better to have a single
>> > critical section.
>>
>> You mean merge the critical region for patching and code-generation?
>
> Yes, I'd keep the lock held and drop it (if it was held) after the patching
> is done, like IIRC we used to do:
> (snip)
>> > I propose to just extend the critical section, like we used to
>> > do with tcg_lock_reset.
Hmm, so I came up with this:
/*
* Patch the last TB with a jump to the current TB.
*
* Modification of the TB has to be protected with tb_lock which can
* either be already held or taken here.
*/
static inline void maybe_patch_last_tb(CPUState *cpu,
TranslationBlock *tb,
TranslationBlock **last_tb,
int tb_exit,
bool locked)
{
if (cpu->tb_flushed) {
/* Ensure that no TB jump will be modified as the
* translation buffer has been flushed.
*/
*last_tb = NULL;
cpu->tb_flushed = false;
}
#ifndef CONFIG_USER_ONLY
/* We don't take care of direct jumps when address mapping changes in
* system emulation. So it's not safe to make a direct jump to a TB
* spanning two pages because the mapping for the second page can change.
*/
if (tb->page_addr[1] != -1) {
*last_tb = NULL;
}
#endif
/* See if we can patch the calling TB. */
if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
if (!locked) {
tb_lock();
}
tb_add_jump(*last_tb, tb_exit, tb);
if (!locked) {
tb_unlock();
}
}
}
/*
* tb_find - find next TB, possibly generating it
*
* There is a multi-level lookup for finding the next TB which avoids
* locks unless generation is required.
*
* 1. Lookup via the per-vcpu tb_jmp_cache
* 2. Lookup via tb_find_physical (using QHT)
*
* If both of those fail then we need to grab the mmap_lock and
* tb_lock and do code generation.
*
* As the jump patching of code also needs to be protected by locks we
* have multiple paths into maybe_patch_last_tb taking advantage of
* the fact we may already have locks held for code generation.
*/
static TranslationBlock *tb_find(CPUState *cpu,
TranslationBlock **last_tb,
int tb_exit)
{
CPUArchState *env = (CPUArchState *)cpu->env_ptr;
TranslationBlock *tb;
target_ulong cs_base, pc;
unsigned int h;
uint32_t flags;
/* we record a subset of the CPU state. It will
always be the same before a given translated block
is executed. */
cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
h = tb_jmp_cache_hash_func(pc);
tb = atomic_read(&cpu->tb_jmp_cache[h]);
if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
tb->flags != flags)) {
/* Ensure that we won't find a TB in the shared hash table
* if it is being invalidated by some other thread.
* Otherwise we'd put it back to CPU's local cache.
* Pairs with smp_wmb() in tb_phys_invalidate(). */
smp_rmb();
tb = tb_find_physical(cpu, pc, cs_base, flags);
if (!tb) {
/* 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();
/* 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);
}
maybe_patch_last_tb(cpu, tb, last_tb, tb_exit, true);
tb_unlock();
mmap_unlock();
} else {
maybe_patch_last_tb(cpu, tb, last_tb, tb_exit, false);
}
/* We update the TB in the virtual pc hash table for the fast lookup */
atomic_set(&cpu->tb_jmp_cache[h], tb);
} else {
maybe_patch_last_tb(cpu, tb, last_tb, tb_exit, false);
}
return tb;
}
But it doesn't seem to make much difference to the microbenchmark and I
think makes the code a lot trickier to follow.
Is it worth it?
--
Alex Bennée
- [Qemu-devel] [PATCH 0/2] Reduce lock contention on TCG hot-path, Alex Bennée, 2016/07/01
- [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/01
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Richard Henderson, 2016/07/01
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Emilio G. Cota, 2016/07/01
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/04
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Emilio G. Cota, 2016/07/04
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path,
Alex Bennée <=
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Paolo Bonzini, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Paolo Bonzini, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Sergey Fedorov, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Alex Bennée, 2016/07/05
- Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path, Sergey Fedorov, 2016/07/05
[Qemu-devel] [PATCH 1/2] tcg: Ensure safe tb_jmp_cache lookup out of 'tb_lock', Alex Bennée, 2016/07/01