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.
