Messages in this thread | | | Date | Thu, 26 Oct 2017 10:11:16 +1100 | From | "Tobin C. Harding" <> | Subject | Re: [PATCH v7] printk: hash addresses printed with %p |
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On Thu, Oct 26, 2017 at 12:59:08AM +0200, Jason A. Donenfeld wrote: > On Thu, Oct 26, 2017 at 12:27 AM, Tobin C. Harding <me@tobin.cc> wrote: > > How good is unlikely()? > > It places that branch way at the bottom of the function so that it's > less likely to pollute the icache. > > > It doesn't _feel_ right adding a check on every call to printk just to > > check for a condition that was only true for the briefest time when the > > kernel booted. But if unlikely() is good then I guess it doesn't hurt. > > > > I'm leaning towards the option 1, but then all those text books I read > > are telling me to implement the simplest solution first then if we need > > to go faster implement the more complex solution. > > > > This is a pretty airy fairy discussion now, but if you have an opinion > > I'd love to hear it. > > I don't think adding a single branch there really matters that much, > considering how many random other branches there are all over the > printk code. However, if you really want to optimize on the little > bits, and sensibly don't want to go with the overcomplex > workqueue-to-statickey thing, you could consider using a plain vanilla > `bool has_gotten_random_ptr_secret` instead of using the atomic_t. The > reason is that there's only ever one single writer, changing from a 0 > to a 1. Basically the only thing doing the atomic_t got you was a > cache flush surrounding the read (and the write) so that assigning > has_gotten_random_ptr_secret=true would take effect _immediately_. > However, since you might not necessarily about that, going with a bool > instead will save you an expensive cache flush, while potentially > being a microsecond out of date the first time it's used. Seems like > an okay trade off to me. (That kind of cache latency, also, is a few > orders of magnitude better than using a work queue for the statickey > stuff.)
Awesome. Patch to follow.
thanks, Tobin.
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