From: Nicholas Piggin <npiggin@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>
Cc: Anton Blanchard <anton@ozlabs.org>,
linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org,
Randy Dunlap <rdunlap@infradead.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable
Date: Mon, 14 Jun 2021 10:45:22 +1000 [thread overview]
Message-ID: <1623629185.fxzl5xdab6.astroid@bobo.none> (raw)
In-Reply-To: <8ac1d420-b861-f586-bacf-8c3949e9b5c4@kernel.org>
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?
> 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.
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
next prev parent reply other threads:[~2021-06-14 0:46 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-05 1:42 [PATCH v4 0/4] shoot lazy tlbs Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 1/4] lazy tlb: introduce lazy mm refcount helper functions Nicholas Piggin
2021-06-07 23:49 ` Andrew Morton
2021-06-08 1:39 ` Nicholas Piggin
2021-06-08 1:48 ` Andrew Morton
2021-06-08 4:11 ` Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable Nicholas Piggin
2021-06-08 3:11 ` Nicholas Piggin
2021-06-08 16:20 ` Andy Lutomirski
2021-06-14 0:45 ` Nicholas Piggin [this message]
2021-06-14 3:52 ` Andy Lutomirski
2021-06-14 4:14 ` Nicholas Piggin
2021-06-14 4:47 ` Nicholas Piggin
2021-06-14 5:21 ` Nicholas Piggin
2021-06-14 16:20 ` Andy Lutomirski
2021-06-15 0:55 ` Nicholas Piggin
2021-06-16 0:14 ` Andy Lutomirski
2021-06-16 1:02 ` Nicholas Piggin
2021-06-17 0:32 ` Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 3/4] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Nicholas Piggin
2021-06-08 3:15 ` Nicholas Piggin
2021-06-05 1:42 ` [PATCH v4 4/4] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN Nicholas Piggin
2021-06-07 23:52 ` Andrew Morton
2021-06-08 2:13 ` Nicholas Piggin
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