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[213.95.11.211]) by mx.google.com with ESMTPS id q4si1869017wrn.99.2019.06.21.01.15.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 21 Jun 2019 01:15:33 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of hch@lst.de designates 213.95.11.211 as permitted sender) client-ip=213.95.11.211; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of hch@lst.de designates 213.95.11.211 as permitted sender) smtp.mailfrom=hch@lst.de Received: by newverein.lst.de (Postfix, from userid 2407) id BAE6368C4E; Fri, 21 Jun 2019 10:15:01 +0200 (CEST) Date: Fri, 21 Jun 2019 10:15:01 +0200 From: Christoph Hellwig To: Linus Torvalds Cc: Nicholas Piggin , 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 Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in a structure Message-ID: <20190621081501.GA17718@lst.de> References: <20190611144102.8848-1-hch@lst.de> <20190611144102.8848-17-hch@lst.de> <1560300464.nijubslu3h.astroid@bobo.none> <1561032202.0qfct43s2c.astroid@bobo.none> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.17 (2007-11-01) 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 10:21:46AM -0700, Linus Torvalds wrote: > 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). I've not seen any difference in the GUP bench output here ar all. But I'm fine with skipping this patch for now, I have a potential series I'm looking into that would benefit a lot from it, but we can discusss it in that context and make sure all the other works gets in in time.