From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Date: Fri, 21 Jun 2019 08:15:01 +0000 Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in a structure Message-Id: <20190621081501.GA17718@lst.de> List-Id: 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: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 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.