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

[Andy Lutomirski]

> 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.

So I think it’s worth at least giving this a try.

> 
> Thanks,
> Nick