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From: Sergey Fedorov <serge.fdrv@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Sergey Fedorov" <sergey.fedorov@linaro.org>,
	qemu-devel@nongnu.org, "Alex Bennée" <alex.bennee@linaro.org>,
	patches@linaro.org, mttcg@greensocs.com,
	"fred konrad" <fred.konrad@greensocs.com>,
	"a rigo" <a.rigo@virtualopensystems.com>,
	cota@braap.org, "bobby prani" <bobby.prani@gmail.com>,
	rth@twiddle.net, "mark burton" <mark.burton@greensocs.com>,
	"jan kiszka" <jan.kiszka@siemens.com>,
	"peter maydell" <peter.maydell@linaro.org>,
	"claudio fontana" <claudio.fontana@huawei.com>,
	"Peter Crosthwaite" <crosthwaite.peter@gmail.com>
Subject: Re: [Qemu-devel] [PATCH 3/3] tcg: Avoid bouncing tb_lock between tb_gen_code() and tb_add_jump()
Date: Fri, 8 Jul 2016 23:24:41 +0300	[thread overview]
Message-ID: <57800C09.30808@gmail.com> (raw)
In-Reply-To: <75bb3f61-0b90-1805-1d21-8e83377e133f@redhat.com>

On 08/07/16 23:18, Paolo Bonzini wrote:
>
> On 08/07/2016 21:55, Sergey Fedorov wrote:
>> On 08/07/16 17:07, Paolo Bonzini wrote:
>>> On 08/07/2016 14:32, Sergey Fedorov wrote:
>>>>>>>> I think we can do even better. One option is using a separate tiny lock
>>>>>>>> to protect direct jump set/reset instead of tb_lock.
>>>>>> If you have to use a separate tiny lock, you don't gain anything compared
>>>>>> to the two critical sections, do you?
>>>> If we have a separate lock for direct jump set/reset then we can do fast
>>>> TB lookup + direct jump patching without taking tb_lock at all. How much
>>>> this would reduce lock contention largely depends on the workload we use.
>>> Yeah, it probably would be easy enough that it's hard to object to it
>>> (unlike the other idea below, which I'm not very comfortable with, at
>>> least without seeing patches).
>>>
>>> The main advantage would be that this tiny lock could be a spinlock
>>> rather than a mutex.
>> Well, the problem is more subtle than we thought: tb_find_fast() can
>> race with tb_phys_invalidate(). The first tb_find_phys() out of the lock
>> can return a TB which is being invalidated. Then a direct jump can be
>> set up to this TB. It can happen after concurrent tb_phys_invalidate()
>> resets all the direct jumps to the TB. Thus we can end up with a direct
>> jump to an invalidated TB. Even extending tb_lock critical section
>> wouldn't help if at least one tb_find_phys() is performed out of the lock.
> Ahem, isn't this exactly why tb_find_phys was invalidating the PC in my
> patches, as the very first step?...  (The smp_wmb after invalidating the
> PC paired with an atomic_rcu_read in tb_find_fast; now we could do it
> after computing the hash and before calling qht_remove).
>
> It turned out that invalidating the PC wasn't as easy as writing -1 to
> the pc, but it's possible to do one of these:
>
> 1) set cs_base to an invalid value (all-ones works for everything except
> x86---instead anything nonzero is enough except on
> x86 and SPARC)
>
> 2) set the flags to an invalid combination (x86 can use all ones or
> rename the useless HF_SOFTMMU_MASK to HF_INVALID_MASK).

I remember, I've just found that we discussed it in this thread:

http://thread.gmane.org/gmane.comp.emulators.qemu/401723/focus=406852

I was thinking of just doing 'tb_jmp_cache' lookup out of the lock, not
tb_find_physical(). Now thanks to QHT, we could do tb_find_physical()
out of the lock, too. This changes things.

Kind regards,
Sergey

  reply	other threads:[~2016-07-08 20:24 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-07-05 16:18 [Qemu-devel] [PATCH v2 0/6] Reduce lock contention on TCG hot-path Alex Bennée
2016-07-05 16:18 ` [Qemu-devel] [PATCH v2 1/6] tcg: Ensure safe tb_jmp_cache lookup out of 'tb_lock' Alex Bennée
2016-07-07 13:52   ` Sergey Fedorov
2016-07-08 14:51   ` Sergey Fedorov
2016-07-05 16:18 ` [Qemu-devel] [PATCH v2 2/6] tcg: set up tb->page_addr before insertion Alex Bennée
2016-07-07 14:08   ` Sergey Fedorov
2016-07-08  9:40     ` Sergey Fedorov
2016-07-05 16:18 ` [Qemu-devel] [PATCH v2 3/6] tcg: cpu-exec: remove tb_lock from the hot-path Alex Bennée
2016-07-07 14:18   ` Sergey Fedorov
2016-07-08 15:50     ` Sergey Fedorov
2016-07-08 17:34     ` Sergey Fedorov
2016-07-08 18:03       ` Alex Bennée
2016-07-08 18:20         ` Sergey Fedorov
2016-07-08 20:09   ` Sergey Fedorov
2016-07-05 16:18 ` [Qemu-devel] [PATCH v2 4/6] tcg: cpu-exec: factor out TB patching code Alex Bennée
2016-07-05 16:18 ` [Qemu-devel] [PATCH v2 5/6] tcg: introduce tb_lock_recursive() Alex Bennée
2016-07-05 16:18 ` [Qemu-devel] [PATCH v2 6/6] tcg: cpu-exec: roll-up tb_find_fast/slow Alex Bennée
2016-07-07 16:44   ` Sergey Fedorov
2016-07-07 16:44     ` [Qemu-devel] [PATCH 1/3] tcg: Introduce mmap_lock_reset() Sergey Fedorov
2016-07-07 16:44     ` [Qemu-devel] [PATCH 2/3] tcg: Introduce tb_lock_locked() Sergey Fedorov
2016-07-07 16:44     ` [Qemu-devel] [PATCH 3/3] tcg: Avoid bouncing tb_lock between tb_gen_code() and tb_add_jump() Sergey Fedorov
2016-07-07 19:36       ` Alex Bennée
2016-07-07 19:46         ` Sergey Fedorov
2016-07-07 20:36           ` Sergey Fedorov
2016-07-07 21:40             ` Alex Bennée
2016-07-08  8:40       ` Paolo Bonzini
2016-07-08 10:25         ` Sergey Fedorov
2016-07-08 11:02           ` Paolo Bonzini
2016-07-08 12:32             ` Sergey Fedorov
2016-07-08 14:07               ` Paolo Bonzini
2016-07-08 19:55                 ` Sergey Fedorov
2016-07-08 20:18                   ` Paolo Bonzini
2016-07-08 20:24                     ` Sergey Fedorov [this message]
2016-07-08 20:52                       ` Paolo Bonzini
2016-07-11 13:06                         ` Sergey Fedorov
2016-07-11 14:03                           ` Paolo Bonzini
2016-07-11 14:27                             ` Sergey Fedorov
2016-07-07 16:04 ` [Qemu-devel] [PATCH v2 0/6] Reduce lock contention on TCG hot-path Emilio G. Cota
2016-07-07 16:13   ` Paolo Bonzini
2016-07-07 19:33     ` Alex Bennée
2016-07-07 19:38   ` Alex Bennée

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