On Jul 13, 2020, at 9:48 AM, Nicholas Piggin [off-list ref] wrote:

Excerpts from Andy Lutomirski's message of July 14, 2020 1:59 am:

quoted

quoted

On Thu, Jul 9, 2020 at 6:57 PM Nicholas Piggin [off-list ref] 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