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

[Nicholas Piggin]

Excerpts from Peter Zijlstra's message of July 10, 2020 7:35 pm:
> On Fri, Jul 10, 2020 at 11:56:46AM +1000, Nicholas Piggin 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.
> 
> That's mighty impressive, however:

Well, it's the usual case of "find a bouncing line and scale up the
machine size until you achieve your desired improvements" :) But we
are looking at some fundamental scalabilities and seeing if we can
improve a few things.

> 
>> +static void shoot_lazy_tlbs(struct mm_struct *mm)
>> +{
>> +	if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) {
>> +		smp_call_function_many(mm_cpumask(mm), do_shoot_lazy_tlb, (void *)mm, 1);
>> +		do_shoot_lazy_tlb(mm);
>> +	}
>> +}
> 
> IIRC you (power) never clear a CPU from that mask, so for other
> workloads I can see this resulting in massive amounts of IPIs.
> 
> For instance, take as many processes as you have CPUs. For each,
> manually walk the task across all CPUs and exit. Again.
> 
> Clearly, that's an extreme, but still...

We do have some issues with that, it does tend to be very self-limiting
though, short lived tasks that can drive lots of exits won't get to run
on a lot of cores.

It's worth keeping an eye on, it may not be too hard to mitigate the IPIs
doing something dumb like collecting a queue of mms before killing a
batch of them.

Thanks,
Nick