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

Andy Lutomirski
2020-07-14T12:46:12Z

> On Jul 13, 2020, at 11:31 PM, Nicholas Piggin <npiggin@gmail.com> 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 <npiggin@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@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.

x86 has this exact problem. At least no more than 64*8 CPUs share the cache line :)

Can your share your benchmark?

>
> Thanks,
> Nick