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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id t5sm10919034pfe.116.2021.06.13.22.21.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Jun 2021 22:22:00 -0700 (PDT) Date: Mon, 14 Jun 2021 15:21:55 +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> <1623645385.u2cqbcn3co.astroid@bobo.none> In-Reply-To: <1623645385.u2cqbcn3co.astroid@bobo.none> MIME-Version: 1.0 Message-Id: <1623647326.0np4yc0lo0.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=R2OCkvoG; spf=pass (imf05.hostedemail.com: domain of npiggin@gmail.com designates 209.85.214.169 as permitted sender) smtp.mailfrom=npiggin@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Stat-Signature: edc3p3d5s387ixapco56csms5rmnots9 X-Rspamd-Queue-Id: A233CE000255 X-Rspamd-Server: rspam06 X-HE-Tag: 1623648111-100768 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Excerpts from Nicholas Piggin's message of June 14, 2021 2:47 pm: > 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 tl= b mm >>>>>> when it is context switched. This can be disabled by architectures t= hat >>>>>> don't require this refcounting if they clean up lazy tlb mms when th= e >>>>>> 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. May= be >>>> 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_= mm", >>> 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 a= re, >>> and a "mm_count" counter that is the number of "lazy" users (ie anonym= ous >>> 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, = and >>>>> x86 bare metal *also* does not need ->active_mm. Under the x86 schem= e, >>>>> 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 you= r=20 >>>> kernel threads are using, even in the unmerged CONFIG_LAZY_TLB=3Dn pat= ch, >>>> 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 = away. >>>=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 confusin= g >>> -- 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. >=20 > 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. >=20 > 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. >=20 > 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. >=20 > 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. Ah, that must be where your confusion is coming from: x86's switch_mm=20 doesn't use prev anywhere, and the reference scheme it is using appears=20 to be under-documented, although vague references in changelogs suggest=20 it has not actually "opted out" of active_mm refcounting. That's understandable, but please redirect your objections to the proper=20 place. git blame suggests 3d28ebceaffab. Thanks, Nick