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[209.85.167.50]) by smtp.gmail.com with ESMTPSA id q4sm31252lje.99.2019.06.20.10.30.11 for (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Thu, 20 Jun 2019 10:30:11 -0700 (PDT) Received: by mail-lf1-f50.google.com with SMTP id y198so3049727lfa.1 for ; Thu, 20 Jun 2019 10:30:11 -0700 (PDT) X-Received: by 2002:ac2:5601:: with SMTP id v1mr51955604lfd.106.1561051322002; Thu, 20 Jun 2019 10:22:02 -0700 (PDT) MIME-Version: 1.0 References: <20190611144102.8848-1-hch@lst.de> <20190611144102.8848-17-hch@lst.de> <1560300464.nijubslu3h.astroid@bobo.none> <1561032202.0qfct43s2c.astroid@bobo.none> In-Reply-To: <1561032202.0qfct43s2c.astroid@bobo.none> From: Linus Torvalds Date: Thu, 20 Jun 2019 10:21:46 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in a structure To: Nicholas Piggin Cc: Andrey Konovalov , Benjamin Herrenschmidt , Rich Felker , "David S. Miller" , Christoph Hellwig , James Hogan , Khalid Aziz , Linux List Kernel Mailing , linux-mips@vger.kernel.org, Linux-MM , linuxppc-dev@lists.ozlabs.org, Linux-sh list , Michael Ellerman , Paul Burton , Paul Mackerras , sparclinux@vger.kernel.org, "the arch/x86 maintainers" , Yoshinori Sato Content-Type: text/plain; charset="UTF-8" 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: On Thu, Jun 20, 2019 at 5:19 AM Nicholas Piggin wrote: > > The processor aliasing problem happens because the struct will > be initialised with stores using one base register (e.g., stack > register), and then same memory is loaded using a different > register (e.g., parameter register). Hmm. Honestly, I've never seen anything like that in any kernel profiles. Compared to the problems I _do_ see (which is usually the obvious cache misses, and locking), it must either be in the noise or it's some problem specific to whatever CPU you are doing performance work on? I've occasionally seen pipeline hiccups in profiles, but it's usually been either some serious glass jaw of the core, or it's been something really stupid we did (or occasionally that the compiler did: one in particular I remember was how there was a time when gcc would narrow stores when it could, so if you set a bit in a word, it would do it with a byte store, and then when you read the whole word afterwards you'd get a major pipeline stall and it happened to show up in some really hot paths). Linus