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

[Nicholas Piggin]

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 <npiggin at gmail.com> 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 <npiggin at gmail.com> 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.

Thanks,
Nick