Re: [PATCH v4 4/4] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN

[Nicholas Piggin]

Excerpts from Andrew Morton's message of June 8, 2021 9:52 am:
> On Sat,  5 Jun 2021 11:42:16 +1000 Nicholas Piggin <npiggin@xxxxxxxxx> wrote:
> 
>> 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.
> 
> Nice.  Do we have a feel for the benefit on any real-world workloads?

Not really unfortunately. I think it's always been a "known" cacheline,
it just showed up badly on will-it-scale tests recently when Anton was
doing a sweep of low hanging scalability issues on big systems.

We have some very big systems running certain in-memory databases that 
get into very high contention conditions on mutexes that push context
switch rates right up and with idle times pretty high, which would get
a lot of parallel context switching between user and idle thread, we
might be getting a bit of this contention there.

It's not something at the top of profiles though. And on multi-threaded
workloads like this, the normal refcounting of the user mm still has
fundmaental contention. It's tricky to get the change tested on these
workloads (machine time is very limited and I can't drive the software).

I suspect it could also show in things that do high net or disk IO rates
(enough to need a lot of cores), and do some user processing steps along
the way. You'd potentially get a lot of idle switching.

> 
> Could any other architectures benefit from these changes?
> 

The cacheline is going to bounce in the same situations on other archs, 
so I would say yes. Rik at one stage had some patches to try avoid it
for x86 some years ago, I don't know what happened to those.

The way powerpc has to maintain mm_cpumask for its TLB flushing makes it
relatively easy to do this shootdown, and we decided the additional IPIs
were less of a concern than the bouncing. Others have different concerns,
but I tried to make it generic and add comments explaining what other
archs can do, or possibly different ways it might be achieved.

Thanks,
Nick