Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design ofPer-CPU Reader-Writer Locks

From: Lai Jiangshan
Date: Tue Feb 26 2013 - 19:33:32 EST

On Wed, Feb 27, 2013 at 3:30 AM, Srivatsa S. Bhat
<srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
> On 02/26/2013 09:55 PM, Lai Jiangshan wrote:
>> On Tue, Feb 26, 2013 at 10:22 PM, Srivatsa S. Bhat
>> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>>> Hi Lai,
>>> I'm really not convinced that piggy-backing on lglocks would help
>>> us in any way. But still, let me try to address some of the points
>>> you raised...
>>> On 02/26/2013 06:29 PM, Lai Jiangshan wrote:
>>>> On Tue, Feb 26, 2013 at 5:02 PM, Srivatsa S. Bhat
>>>> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>>>>> On 02/26/2013 05:47 AM, Lai Jiangshan wrote:
>>>>>> On Tue, Feb 26, 2013 at 3:26 AM, Srivatsa S. Bhat
>>>>>> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>>>>>>> Hi Lai,
>>>>>>> On 02/25/2013 09:23 PM, Lai Jiangshan wrote:
>>>>>>>> Hi, Srivatsa,
>>>>>>>> The target of the whole patchset is nice for me.
>>>>>>> Cool! Thanks :-)
>>>>> [...]
>>>>> Unfortunately, I see quite a few issues with the code above. IIUC, the
>>>>> writer and the reader both increment the same counters. So how will the
>>>>> unlock() code in the reader path know when to unlock which of the locks?
>>>> The same as your code, the reader(which nested in write C.S.) just dec
>>>> the counters.
>>> And that works fine in my case because the writer and the reader update
>>> _two_ _different_ counters.
>> I can't find any magic in your code, they are the same counter.
>> /*
>> * It is desirable to allow the writer to acquire the percpu-rwlock
>> * for read (if necessary), without deadlocking or getting complaints
>> * from lockdep. To achieve that, just increment the reader_refcnt of
>> * this CPU - that way, any attempt by the writer to acquire the
>> * percpu-rwlock for read, will get treated as a case of nested percpu
>> * reader, which is safe, from a locking perspective.
>> */
>> this_cpu_inc(pcpu_rwlock->rw_state->reader_refcnt);
> Whoa! Hold on, were you really referring to _this_ increment when you said
> that, in your patch you would increment the refcnt at the writer? Then I guess
> there is a major disconnect in our conversations. (I had assumed that you were
> referring to the update of writer_signal, and were just trying to have a single
> refcnt instead of reader_refcnt and writer_signal).

Sorry the name "fallback_reader_refcnt" misled you.

> So, please let me clarify things a bit here. Forget about the above increment
> of reader_refcnt at the writer side. Its almost utterly insignificant for our
> current discussion. We can simply replace it with a check as shown below, at
> the reader side:
> void percpu_read_lock_irqsafe()
> {
> if (current == active_writer)
> return;
> /* Rest of the code */
> }
> Now, assuming that, in your patch, you were trying to use the per-cpu refcnt
> to allow the writer to safely take the reader path, you can simply get rid of
> that percpu-refcnt, as demonstrated above.
> So that would reduce your code to the following (after simplification):
> lg_rwlock_local_read_lock()
> {
> if (current == active_writer)
> return;
> if (arch_spin_trylock(per-cpu-spinlock))
> return;
> read_lock(global-rwlock);
> }
> Now, let us assume that hotplug is not happening, meaning, nobody is running
> the writer side code. Now let's see what happens at the reader side in your
> patch. As I mentioned earlier, the readers are _very_ frequent and can be in
> very hot paths. And they also happen to be nested quite often.
> So, a non-nested reader acquires the per-cpu spinlock. Every subsequent nested
> reader on that CPU has to acquire the global rwlock for read. Right there you
> have 2 significant performance issues -
> 1. Acquiring the (spin) lock is costly
> 2. Acquiring the global rwlock causes cacheline bouncing, which hurts
> performance.
> And why do we care so much about performance here? Because, the existing
> kernel does an efficient preempt_disable() here - which is an optimized
> per-cpu counter increment. Replacing that with such heavy primitives on the
> reader side can be very bad.
> Now, how does my patchset tackle this? My scheme just requires an increment
> of a per-cpu refcnt (reader_refcnt) and memory barrier. Which is acceptable
> from a performance-perspective, because IMHO its not horrendously worse than
> a preempt_disable().
>>> If both of them update the same counter, there
>>> will be a semantic clash - an increment of the counter can either mean that
>>> a new writer became active, or it can also indicate a nested reader. A decrement
>>> can also similarly have 2 meanings. And thus it will be difficult to decide
>>> the right action to take, based on the value of the counter.
>>>>> (The counter-dropping-to-zero logic is not safe, since it can be updated
>>>>> due to different reasons). And now that I look at it again, in the absence
>>>>> of the writer, the reader is allowed to be recursive at the heavy cost of
>>>>> taking the global rwlock for read, every 2nd time you nest (because the
>>>>> spinlock is non-recursive).
>>>> (I did not understand your comments of this part)
>>>> nested reader is considered seldom.
>>> No, nested readers can be _quite_ frequent. Because, potentially all users
>>> of preempt_disable() are readers - and its well-known how frequently we
>>> nest preempt_disable(). As a simple example, any atomic reader who calls
>>> smp_call_function() will become a nested reader, because smp_call_function()
>>> itself is a reader. So reader nesting is expected to be quite frequent.
>>>> But if N(>=2) nested readers happen,
>>>> the overhead is:
>>>> 1 spin_try_lock() + 1 read_lock() + (N-1) __this_cpu_inc()
>>> In my patch, its just this_cpu_inc(). Note that these are _very_ hot paths.
>>> So every bit of optimization that you can add is worthwhile.
>>> And your read_lock() is a _global_ lock - thus, it can lead to a lot of
>>> cache-line bouncing. That's *exactly* why I have used per-cpu refcounts in
>>> my synchronization scheme, to avoid taking the global rwlock as much as possible.
>>> Another important point to note is that, the overhead we are talking about
>>> here, exists even when _not_ performing hotplug. And its the replacement to
>>> the super-fast preempt_disable(). So its extremely important to consciously
>>> minimize this overhead - else we'll end up slowing down the system significantly.
>> All I was considered is "nested reader is seldom", so I always
>> fallback to rwlock when nested.
>> If you like, I can add 6 lines of code, the overhead is
>> 1 spin_try_lock()(fast path) + N __this_cpu_inc()
> I'm assuming that calculation is no longer valid, considering that
> we just discussed how the per-cpu refcnt that you were using is quite
> unnecessary and can be removed.
> IIUC, the overhead with your code, as per above discussion would be:
> 1 spin_try_lock() [non-nested] + N read_lock(global_rwlock).

Again, I'm so sorry the name "fallback_reader_refcnt" misled you.

> Note that I'm referring to the scenario when hotplug is _not_ happening
> (ie., nobody is running writer side code).
>> The overhead of your code is
>> 2 smp_mb() + N __this_cpu_inc()
> Right. And as you can see, this is much much better than the overhead
> shown above.

I will write a test to compare it to "1 spin_try_lock()(fast path) +
N __this_cpu_inc()"

>> I don't see how much different.
>>>>> Also, this lg_rwlock implementation uses 3
>>>>> different data-structures - a per-cpu spinlock, a global rwlock and
>>>>> a per-cpu refcnt, and its not immediately apparent why you need those many
>>>>> or even those many varieties.
>>>> data-structures is the same as yours.
>>>> fallback_reader_refcnt <--> reader_refcnt
>>> This has semantic problems, as noted above.
>>>> per-cpu spinlock <--> write_signal
>>> Acquire/release of (spin) lock is costlier than inc/dec of a counter, IIUC.
>>>> fallback_rwlock <---> global_rwlock
>>>>> Also I see that this doesn't handle the
>>>>> case of interrupt-handlers also being readers.
>>>> handled. nested reader will see the ref or take the fallback_rwlock
>> Sorry, _reentrance_ read_lock() will see the ref or take the fallback_rwlock
>>> I'm not referring to simple nested readers here, but interrupt handlers who
>>> can act as readers. For starters, the arch_spin_trylock() is not safe when
>>> interrupt handlers can also run the same code, right? You'll need to save
>>> and restore interrupts at critical points in the code. Also, the __foo()
>>> variants used to read/update the counters are not interrupt-safe.
>> I must missed something.
>> Could you elaborate more why arch_spin_trylock() is not safe when
>> interrupt handlers can also run the same code?
>> Could you elaborate more why __this_cpu_op variants is not
>> interrupt-safe since they are always called paired.
> Take a look at include/linux/percpu.h. You'll note that __this_cpu_*
> operations map to __this_cpu_generic_to_op(), which doesn't disable interrupts
> while doing the update. Hence you can get inconsistent results if an interrupt
> hits the CPU at that time and the interrupt handler tries to do the same thing.
> In contrast, if you use this_cpu_inc() for example, interrupts are explicitly
> disabled during the update and hence you won't get inconsistent results.

xx_lock()/xx_unlock() are must called paired, if interrupts happens
the value of the data is recovered after the interrupts return.

the same reason, preempt_disable() itself it is not irqsafe,
but preempt_disable()/preempt_enable() are called paired, so they are
all irqsafe.

>>> And,
>>> the unlock() code in the reader path is again going to be confused about
>>> what to do when interrupt handlers interrupt regular readers, due to the
>>> messed up refcount.
>> I still can't understand.
> The primary reason _I_ was using the refcnt vs the reason _you_ were using the
> refcnt, appears to be very different. Maybe that's why the above statement
> didn't make sense. In your case, IIUC, you can simply get rid of the refcnt
> and replace it with the simple check I mentioned above. Whereas, I use
> refcnts to keep the reader-side synchronization fast (and for reader-writer
> communication).
> Regards,
> Srivatsa S. Bhat
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