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

[Nicholas Piggin]

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.

Thanks,
Nick