Subject: Re: [RFC PATCH 7/7] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

From: Nicholas Piggin <[email-masked]>
Date: Thu, 16 Jul 2020 12:35:12 +1000

Excerpts from Andy Lutomirski's message of July 14, 2020 10:46 pm:
> 
> 
>> On Jul 13, 2020, at 11:31 PM, Nicholas Piggin <[email-masked]> wrote:
>> 
>> Excerpts from Nicholas Piggin's message of July 14, 2020 3:04 pm:
>>> Excerpts from Andy Lutomirski's message of July 14, 2020 4:18 am:
>>>> 
>>>>> On Jul 13, 2020, at 9:48 AM, Nicholas Piggin <[email-masked]> wrote:
>>>>> 
>>>>> Excerpts from Andy Lutomirski's message of July 14, 2020 1:59 am:
>>>>>>> On Thu, Jul 9, 2020 at 6:57 PM Nicholas Piggin <[email-masked]> wrote:
>>>>>>> 
>>>>>>> On big systems, the mm refcount can become highly contented when doing
>>>>>>> a lot of context switching with threaded applications (particularly
>>>>>>> switching between the idle thread and an application thread).
>>>>>>> 
>>>>>>> Abandoning lazy tlb slows switching down quite a bit in the important
>>>>>>> user->idle->user cases, so so instead implement a non-refcounted scheme
>>>>>>> that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down
>>>>>>> any remaining lazy ones.
>>>>>>> 
>>>>>>> On a 16-socket 192-core POWER8 system, a context switching benchmark
>>>>>>> with as many software threads as CPUs (so each switch will go in and
>>>>>>> out of idle), upstream can achieve a rate of about 1 million context
>>>>>>> switches per second. After this patch it goes up to 118 million.
>>>>>>> 
>>>>>> 
>>>>>> I read the patch a couple of times, and I have a suggestion that could
>>>>>> be nonsense.  You are, effectively, using mm_cpumask() as a sort of
>>>>>> refcount.  You're saying "hey, this mm has no more references, but it
>>>>>> still has nonempty mm_cpumask(), so let's send an IPI and shoot down
>>>>>> those references too."  I'm wondering whether you actually need the
>>>>>> IPI.  What if, instead, you actually treated mm_cpumask as a refcount
>>>>>> for real?  Roughly, in __mmdrop(), you would only free the page tables
>>>>>> if mm_cpumask() is empty.  And, in the code that removes a CPU from
>>>>>> mm_cpumask(), you would check if mm_users == 0 and, if so, check if
>>>>>> you just removed the last bit from mm_cpumask and potentially free the
>>>>>> mm.
>>>>>> 
>>>>>> Getting the locking right here could be a bit tricky -- you need to
>>>>>> avoid two CPUs simultaneously exiting lazy TLB and thinking they
>>>>>> should free the mm, and you also need to avoid an mm with mm_users
>>>>>> hitting zero concurrently with the last remote CPU using it lazily
>>>>>> exiting lazy TLB.  Perhaps this could be resolved by having mm_count
>>>>>> == 1 mean "mm_cpumask() is might contain bits and, if so, it owns the
>>>>>> mm" and mm_count == 0 meaning "now it's dead" and using some careful
>>>>>> cmpxchg or dec_return to make sure that only one CPU frees it.
>>>>>> 
>>>>>> Or maybe you'd need a lock or RCU for this, but the idea would be to
>>>>>> only ever take the lock after mm_users goes to zero.
>>>>> 
>>>>> I don't think it's nonsense, it could be a good way to avoid IPIs.
>>>>> 
>>>>> I haven't seen much problem here that made me too concerned about IPIs 
>>>>> yet, so I think the simple patch may be good enough to start with
>>>>> for powerpc. I'm looking at avoiding/reducing the IPIs by combining the
>>>>> unlazying with the exit TLB flush without doing anything fancy with
>>>>> ref counting, but we'll see.
>>>> 
>>>> I would be cautious with benchmarking here. I would expect that the
>>>> nasty cases may affect power consumption more than performance — the 
>>>> specific issue is IPIs hitting idle cores, and the main effects are to 
>>>> slow down exit() a bit but also to kick the idle core out of idle. 
>>>> Although, if the idle core is in a deep sleep, that IPI could be 
>>>> *very* slow.
>>> 
>>> It will tend to be self-limiting to some degree (deeper idle cores
>>> would tend to have less chance of IPI) but we have bigger issues on
>>> powerpc with that, like broadcast IPIs to the mm cpumask for THP
>>> management. Power hasn't really shown up as an issue but powerpc
>>> CPUs may have their own requirements and issues there, shall we say.
>>> 
>>>> So I think it’s worth at least giving this a try.
>>> 
>>> To be clear it's not a complete solution itself. The problem is of 
>>> course that mm cpumask gives you false negatives, so the bits
>>> won't always clean up after themselves as CPUs switch away from their
>>> lazy tlb mms.
>> 
>> ^^
>> 
>> False positives: CPU is in the mm_cpumask, but is not using the mm
>> as a lazy tlb. So there can be bits left and never freed.
>> 
>> If you closed the false positives, you're back to a shared mm cache
>> line on lazy mm context switches.
> 
> x86 has this exact problem. At least no more than 64*8 CPUs share the cache line :)
> 
> Can your share your benchmark?

Just testing the IPI rates (on a smaller 176 CPU system), on a
kernel compile, it causes about 300 shootdown interrupts (not
300 broadcasts but total interrupts).

And very short lived fork;exec;exit things like typical scripting
commands doesn't typically generate any.

So yeah the really high exit rate things self-limit pretty well.

I documented the concern and added a few of the possible ways to
further reduce IPIs in the comments though.

Thanks,
Nick