From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nicholas Piggin Date: Fri, 21 Jun 2019 23:55:09 +0000 Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in a structure Message-Id: <1561160786.mradw6fg2v.astroid@bobo.none> 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> <20190621081501.GA17718@lst.de> In-Reply-To: <20190621081501.GA17718@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Christoph Hellwig , Linus Torvalds Cc: Andrey Konovalov , Benjamin Herrenschmidt , Rich Felker , "David S. Miller" , 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 Christoph Hellwig's on June 21, 2019 6:15 pm: > 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. > If you can, that would be good. I don't like to object based on handwaving so I'll see if I can find any benchmarks that will give better confidence. Those old TPC-C tests were good, and there was some DB2 workload that was the reason I added gup fast in the first place. I'll do some digging. Thanks, Nick