Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable

[Andy Lutomirski <luto@kernel.org>]

On 6/13/21 5:45 PM, Nicholas Piggin wrote:
> Excerpts from Andy Lutomirski's message of June 9, 2021 2:20 am:
>> On 6/4/21 6:42 PM, Nicholas Piggin wrote:
>>> Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb mm
>>> when it is context switched. This can be disabled by architectures that
>>> don't require this refcounting if they clean up lazy tlb mms when the
>>> last refcount is dropped. Currently this is always enabled, which is
>>> what existing code does, so the patch is effectively a no-op.
>>>
>>> Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is.
>>
>> I am in favor of this approach, but I would be a lot more comfortable
>> with the resulting code if task->active_mm were at least better
>> documented and possibly even guarded by ifdefs.
> 
> active_mm is fairly well documented in Documentation/active_mm.rst IMO.
> I don't think anything has changed in 20 years, I don't know what more
> is needed, but if you can add to documentation that would be nice. Maybe
> moving a bit of that into .c and .h files?
> 

Quoting from that file:

  - however, we obviously need to keep track of which address space we
    "stole" for such an anonymous user. For that, we have "tsk->active_mm",
    which shows what the currently active address space is.

This isn't even true right now on x86.  With your patch applied:

 To support all that, the "struct mm_struct" now has two counters: a
 "mm_users" counter that is how many "real address space users" there are,
 and a "mm_count" counter that is the number of "lazy" users (ie anonymous
 users) plus one if there are any real users.

isn't even true any more.


>> x86 bare metal currently does not need the core lazy mm refcounting, and
>> x86 bare metal *also* does not need ->active_mm.  Under the x86 scheme,
>> if lazy mm refcounting were configured out, ->active_mm could become a
>> dangling pointer, and this makes me extremely uncomfortable.
>>
>> So I tend to think that, depending on config, the core code should
>> either keep ->active_mm [1] alive or get rid of it entirely.
> 
> I don't actually know what you mean.
> 
> core code needs the concept of an "active_mm". This is the mm that your 
> kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=n patch,
> active_mm still points to init_mm for kernel threads.

Core code does *not* need this concept.  First, it's wrong on x86 since
at least 4.15.  Any core code that actually assumes that ->active_mm is
"active" for any sensible definition of the word active is wrong.
Fortunately there is no such code.

I looked through all active_mm references in core code.  We have:

kernel/sched/core.c: it's all refcounting, although it's a bit tangled
with membarrier.

kernel/kthread.c: same.  refcounting and membarrier stuff.

kernel/exit.c: exit_mm() a BUG_ON().

kernel/fork.c: initialization code and a warning.

kernel/cpu.c: cpu offline stuff.  wouldn't be needed if active_mm went away.

fs/exec.c: nothing of interest

I didn't go through drivers, but I maintain my point.  active_mm is
there for refcounting.  So please don't just make it even more confusing
-- do your performance improvement, but improve the code at the same
time: get rid of active_mm, at least on architectures that opt out of
the refcounting.



> 
> We could hide that idea behind an active_mm() function that would always 
> return &init_mm if mm==NULL, but you still have the concept of an active
> mm and a pointer that callers must not access after free (because some
> cases will be CONFIG_LAZY_TLB=y).
> 
>> [1] I don't really think it belongs in task_struct at all.  It's not a
>> property of the task.  It's the *per-cpu* mm that the core code is
>> keeping alive for lazy purposes.  How about consolidating it with the
>> copy in rq?
> 
> I agree it's conceptually a per-cpu property. I don't know why it was 
> done this way, maybe it was just convenient and works well for mm and 
> active_mm to be adjacent. Linus might have a better insight.
> 
>> I guess the short summary of my opinion is that I like making this
>> configurable, but I do not like the state of the code.
> 
> I don't think I'd object to moving active_mm to rq and converting all
> usages to active_mm() while we're there, it would make things a bit
> more configurable. But I don't see it making core code fundamentally
> less complex... if you're referring to the x86 mm switching monstrosity,
> then that's understandable, but I admit I haven't spent enough time
> looking at it to make a useful comment. A patch would be enlightening,
> I have the leftover CONFIG_LAZY_TLB=n patch if you were thinking of 
> building on that I can send it to you.
> 
> Thanks,
> Nick
>