On 12/6/21 1:31 AM, Peter Zijlstra wrote: > On Sun, Dec 05, 2021 at 10:53:49PM -0800, Christoph Hellwig wrote: >> On Fri, Dec 03, 2021 at 08:35:40AM -0700, Jens Axboe wrote: >>> refcount_t is not as expensive as it used to be, but it's still more >>> expensive than the io_uring method of using atomic_t and just checking >>> for potential over/underflow. >>> >>> This borrows that same implementation, which in turn is based on the >>> mm implementation from Linus. >> >> If refcount_t isn't good enough for a normal kernel fast path we have >> a problem. Can we discuss that with the maintainers instead of coming >> up with our home grown schemes again? > > Quite; and for something that pretends to be about performance, it also > lacks any actual numbers to back that claim. I can certainly generate that, it was already done for the two previous similar conversions though. > The proposed implementation also doesn't do nearly as much as the > refcount_t one does. > > Anyway refcount_t is just a single "lock xadd" and a few branches, where > does it go wrong? Do you have perf output to compare between them? I'll generate that. -- Jens Axboe
