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[60.242.147.73]) by smtp.gmail.com with ESMTPSA id fw16sm15749448pjb.30.2021.06.13.21.14.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Jun 2021 21:14:32 -0700 (PDT) Date: Mon, 14 Jun 2021 14:14:27 +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> In-Reply-To: <02e16a2f-2f58-b4f2-d335-065e007bcea2@kernel.org> MIME-Version: 1.0 Message-Id: <1623643443.b9twp3txmw.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=gmail.com header.s=20161025 header.b=NojfaKJj; spf=pass (imf07.hostedemail.com: domain of npiggin@gmail.com designates 209.85.215.175 as permitted sender) smtp.mailfrom=npiggin@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Stat-Signature: zfz66sr6oc7wc9g9jnoocs37se6ngdz9 X-Rspamd-Queue-Id: A5645A00014E X-Rspamd-Server: rspam06 X-HE-Tag: 1623644065-615253 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 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 tha= t >>>> 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. Maybe >> 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. >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. Is x86's scheme documented? > 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 are= , > and a "mm_count" counter that is the number of "lazy" users (ie anonymou= s > users) plus one if there are any real users. >=20 > isn't even true any more. Well yeah but the active_mm concept hasn't changed. The refcounting=20 change is hopefully reasonably documented? >=20 >=20 >>> x86 bare metal currently does not need the core lazy mm refcounting, an= d >>> 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 patch= , >> 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 aw= ay. >=20 > fs/exec.c: nothing of interest 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. The part I don't understand is when you say it can just go away. How?=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. powerpc opts out of the refcounting and can not "get rid of active_mm". Not even in theory. Thanks, Nick