Messages in this thread | | | Date | Mon, 29 Nov 2021 09:50:23 +0100 | From | Peter Zijlstra <> | Subject | Re: [GIT pull] locking/urgent for v5.16-rc3 |
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On Sun, Nov 28, 2021 at 09:15:10AM -0800, Linus Torvalds wrote: > On Sun, Nov 28, 2021 at 8:35 AM Thomas Gleixner <tglx@linutronix.de> wrote: > > > > - down_read_trylock() is suboptimal when the lock is contended and > > multiple readers trylock concurrently. That's due to the initial value > > being read non-atomically which results in at least two compare exchange > > loops. Making the initial readout atomic reduces this significantly. > > Whith 40 readers by 11% in a benchmark which enforces contention on > > mmap_sem. > > This was an intentional optimization to avoid unnecessary cache > protocol cycles for when the lock isn't contended - first getting a > cacheline for read ownership, only to then get it for write. > > But I guess we don't have any good benchmarks for non-contention, so ... > > I also hope that maybe modern hardware is smart enough to see "I will > write to it later" and avoid the "get line for shared access only to > get it for exclusive access immediately afterwards" issue.
Yes, I raised that same point, otoh those numbers are not showing that. They did lightly contended, but I suppose not cache-cold.
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