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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id mr23sm11061887pjb.12.2021.06.13.21.47.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Jun 2021 21:47:26 -0700 (PDT) Date: Mon, 14 Jun 2021 14:47:21 +1000 From: Nicholas Piggin Subject: Re: [PATCH v4 2/4] lazy tlb: allow lazy tlb mm refcounting to be configurable To: Andrew Morton , Andy Lutomirski Cc: Anton Blanchard , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, Randy Dunlap , Linus Torvalds References: <20210605014216.446867-1-npiggin@gmail.com> <20210605014216.446867-3-npiggin@gmail.com> <8ac1d420-b861-f586-bacf-8c3949e9b5c4@kernel.org> <1623629185.fxzl5xdab6.astroid@bobo.none> <02e16a2f-2f58-b4f2-d335-065e007bcea2@kernel.org> <1623643443.b9twp3txmw.astroid@bobo.none> In-Reply-To: <1623643443.b9twp3txmw.astroid@bobo.none> MIME-Version: 1.0 Message-Id: <1623645385.u2cqbcn3co.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Excerpts from Nicholas Piggin's message of June 14, 2021 2:14 pm: > Excerpts from Andy Lutomirski's message of June 14, 2021 1:52 pm: >> 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 th= at >>>>> 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. >>>=20 >>> 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. Mayb= e >>> moving a bit of that into .c and .h files? >>>=20 >>=20 >> Quoting from that file: >>=20 >> - however, we obviously need to keep track of which address space we >> "stole" for such an anonymous user. For that, we have "tsk->active_m= m", >> which shows what the currently active address space is. >>=20 >> This isn't even true right now on x86. >=20 > From the perspective of core code, it is. x86 might do something crazy=20 > with it, but it has to make it appear this way to non-arch code that > uses active_mm. >=20 > Is x86's scheme documented? >=20 >> With your patch applied: >>=20 >> 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 ar= e, >> and a "mm_count" counter that is the number of "lazy" users (ie anonymo= us >> users) plus one if there are any real users. >>=20 >> isn't even true any more. >=20 > Well yeah but the active_mm concept hasn't changed. The refcounting=20 > change is hopefully reasonably documented? >=20 >>=20 >>=20 >>>> x86 bare metal currently does not need the core lazy mm refcounting, a= nd >>>> 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. >>>=20 >>> I don't actually know what you mean. >>>=20 >>> core code needs the concept of an "active_mm". This is the mm that your= =20 >>> kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=3Dn patc= h, >>> active_mm still points to init_mm for kernel threads. >>=20 >> 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. >>=20 >> I looked through all active_mm references in core code. We have: >>=20 >> kernel/sched/core.c: it's all refcounting, although it's a bit tangled >> with membarrier. >>=20 >> kernel/kthread.c: same. refcounting and membarrier stuff. >>=20 >> kernel/exit.c: exit_mm() a BUG_ON(). >>=20 >> kernel/fork.c: initialization code and a warning. >>=20 >> kernel/cpu.c: cpu offline stuff. wouldn't be needed if active_mm went a= way. >>=20 >> fs/exec.c: nothing of interest >=20 > I might not have been clear. Core code doesn't need active_mm if=20 > active_mm somehow goes away. I'm saying active_mm can't go away because > it's needed to support (most) archs that do lazy tlb mm switching. >=20 > The part I don't understand is when you say it can just go away. How?=20 >=20 >> 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. >=20 > powerpc opts out of the refcounting and can not "get rid of active_mm". > Not even in theory. That is to say, it does do a type of reference management that requires=20 active_mm so you can argue it has not entirely opted out of refcounting. But we're not just doing refcounting for the sake of refcounting! That would make no sense. active_mm is required because that's the mm that we have switched to=20 (from core code's perspective), and it is integral to know when to=20 switch to a different mm. See how active_mm is a fundamental concept in core code? It's part of the contract between core code and the arch mm context management calls. reference counting follows from there but it's not the _reason_ for this code. Pretend the reference problem does not exit (whether by refcounting or=20 shootdown or garbage collection or whatever). We still can't remove=20 active_mm! We need it to know how to call into arch functions like=20 switch_mm. I don't know if you just forgot that critical requirement in your above=20 list, or you actually are entirely using x86's mental model for this=20 code which is doing something entirely different that does not need it=20 at all. If that is the case I really don't mind some cleanup or wrapper=20 functions for x86 do entirely do its own thing, but if that's the case you can't criticize core code's use of active_mm due to the current state of x86. It's x86 that needs documentation and cleaning up. Thanks, Nick