> From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > Sent: Wednesday, 22 March 2023 15.22 > > On Wed, Mar 22, 2023 at 12:28:44PM +0100, Morten Brørup wrote: > > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > > > Sent: Friday, 17 March 2023 22.49 > > > > > > On Fri, Mar 17, 2023 at 02:42:26PM -0700, Stephen Hemminger wrote: > > > > On Fri, 17 Mar 2023 13:19:41 -0700 > > > > Tyler Retzlaff <roret...@linux.microsoft.com> wrote: > > > > > > > > > Replace the use of rte_atomic.h types and functions, instead use GCC > > > > > supplied C++11 memory model builtins. > > > > > > > > > > This series covers the libraries and drivers that are built on > Windows. > > > > > > > > > > The code has be converted to use the __atomic builtins but there are > > > > > additional during conversion i notice that there may be some issues > > > > > that need to be addressed. > > > > > > > > I don't think all these cmpset need to use SEQ_CST. > > > > Especially for the places where it is used a loop, might > > > > be more efficient with some of the other memory models. > > > > > > i agree. > > > > > > however, i'm not trying to improve the code with this change, just > > > decouple it from rte_atomics.h so trying my best to avoid any > > > unnecessary semantic change. > > > > > > certainly if the maintainers of this code wish to weaken the ordering > > > where appropriate after the change is merged they can do so and handily > > > this change has enabled them to do so easily allowing them to test just > > > their change in isolation. > > > > I agree with the two-step approach, where this first step is a simple > search-and-replacement; but I insist that you add a FIXME or similar note > where you have blindly used SEQ_CST, indicating that the memory order needs to > be reviewed and potentially corrected. > > i think the maintainers need to take some responsibility, if they see > optimizations they missed when previously writing the code they need to > follow up with a patch themselves. i can't do everything for them and > marking things i'm not sure about will only lead to me having to churn > patch series to remove the unwanted comments later.
The previous atomic functions didn't have the "memory order" parameter, so the maintainers didn't have to think about it - and thus they didn't miss any optimizations when accepting the code. I also agree 100 % that it is not your responsibility to consider or determine which memory order is appropriate! But I think you should mark the locations where you are changing from the old rte_atomic functions (where no memory order optimization was available) to the new functions - to highlight where the option of memory ordering has been introduced and knowingly ignored (by you). > > keep in mind i have to touch each of these again when converting to > standard so that's a better time to review ~everything in more detail > because when converting to standard that's when suddenly you get a bunch > of code generation that is "fallback" to seq_cst that isn't happening now. I think you should to do it when replacing the rte_atomic functions with the __atomic functions. It will make it easier to see where the memory order was knowingly ignored, and should be reviewed for optimization. > > the series that converts to standard needs to be up for review as soon > as possible to maximize available time for feedback before 23.11 so it > would be better to get the simpler cut & paste normalizing the code out > of the way to unblock that series submission. > > > > > Also, in a couple of the drivers, you are using int64_t for packet counters. > These cannot be negative and should be uint64_t. And AFAIK, such counters can > use RELAXED memory order. > > i know you don't mean to say i selected the types and rather that the > types that were selected are not quite correct for their usage. Yes; the previous types were also signed, and you didn't change that. > again > on the review that actually adopts std atomics is a better place to make > any potential type changes since we are "breaking" the API for 23.11 > anyway. further, the std atomics series technically changes all the > types so it's probably better to make one type change then rather than > one now and one later. > > i think it would be best to get these validated and merged asap so we > can get to the std atomics review. when that series is up let's discuss > further how i can mark areas of concern, with that series i expect there > will have to be some changes in order to avoid minor regressions. > > thanks! I thought it would be better to catch these details (i.e. memory ordering and signedness) early on, but I now understand that you planned to do it in a later step. So I'll let you proceed as you have planned. Thanks for all your work on this, Tyler. It is much appreciated! -Morten
