Messages in this thread | | | Date | Thu, 15 Apr 2021 10:22:12 +0100 | From | Will Deacon <> | Subject | Re: [RFC][PATCH] locking: Generic ticket-lock |
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On Thu, Apr 15, 2021 at 10:02:18AM +0100, Catalin Marinas wrote: > (fixed Will's email address) > > On Thu, Apr 15, 2021 at 10:09:54AM +0200, Peter Zijlstra wrote: > > On Thu, Apr 15, 2021 at 05:47:34AM +0900, Stafford Horne wrote: > > > > How's this then? Compile tested only on openrisc/simple_smp_defconfig. > > > > > > I did my testing with this FPGA build SoC: > > > > > > https://github.com/stffrdhrn/de0_nano-multicore > > > > > > Note, the CPU timer sync logic uses mb() and is a bit flaky. So missing mb() > > > might be a reason. I thought we had defined mb() and l.msync, but it seems to > > > have gotten lost. > > > > > > With that said I could test out this ticket-lock implementation. How would I > > > tell if its better than qspinlock? > > > > Mostly if it isn't worse, it's better for being *much* simpler. As you > > can see, the guts of ticket is like 16 lines of C (lock+unlock) and you > > only need the behaviour of atomic_fetch_add() to reason about behaviour > > of the whole thing. qspinlock OTOH is mind bending painful to reason > > about. > > > > There are some spinlock tests in locktorture; but back when I had a > > userspace copy of the lot and would measure min,avg,max acquire times > > under various contention loads (making sure to only run a single task > > per CPU etc.. to avoid lock holder preemption and other such 'fun' > > things). > > > > It took us a fair amount of work to get qspinlock to compete with ticket > > for low contention cases (by far the most common in the kernel), and it > > took a fairly large amount of CPUs for qspinlock to really win from > > ticket on the contended case. Your hardware may vary. In particular the > > access to the external cacheline (for queueing, see the queue: label in > > queued_spin_lock_slowpath) is a pain-point and the relative cost of > > cacheline misses for your arch determines where (and if) low contention > > behaviour is competitive. > > > > Also, less variance (the reason for the min/max measure) is better. > > Large variance is typically a sign of fwd progress trouble. > > IIRC, one issue we had with ticket spinlocks on arm64 was on big.LITTLE > systems where the little CPUs were always last to get a ticket when > racing with the big cores. That was with load/store exclusives (LR/SC > style) and would have probably got better with atomics but we moved to > qspinlocks eventually (the Juno board didn't have atomics). > > (leaving the rest of the text below for Will's convenience)
Yes, I think it was this thread:
https://lore.kernel.org/lkml/alpine.DEB.2.20.1707261548560.2186@nanos
but I don't think you can really fix such hardware by changing the locking algorithm (although my proposed cpu_relax() hack was worryingly effective).
Will
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