On Thu, Mar 07, 2019 at 05:35:46PM -0800, Vineet Gupta wrote: > - ARCv2 LLSC based spinlocks smp_mb() both before and after the LLSC > instructions, which is not required per lkmm ACQ/REL semantics. > smp_mb() is only needed _after_ lock and _before_ unlock. > So remove the extra barriers. > The reason they were there was mainly historical. At the time of > initial SMP Linux bringup on HS38 cores, I was too conservative, > given the fluidity of both hw and sw. The last attempt to ditch the > extra barrier showed some hackbench regression which is apparently > not the case now (atleast for LLSC case, read on...) > > - EX based spinlocks (!CONFIG_ARC_HAS_LLSC) still needs the extra > smp_mb(), not due to lkmm, but due to some hardware shenanigans. > W/o that, hackbench triggers RCU stall splat. This is not a "real" > Linux use case anyways so I'm not worried about it. > > | [ARCLinux]# for i in (seq 1 1 5) ; do hackbench; done > | Running with 10 groups 400 process > | INFO: task hackbench:158 blocked for more than 10 seconds. > | Not tainted 4.20.0-00005-g96b18288a88e-dirty #117 > | "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > | hackbench D 0 158 135 0x00000000 > | > | Stack Trace: > | watchdog: BUG: soft lockup - CPU#3 stuck for 59s! [hackbench:469] > | Modules linked in: > | Path: (null) > | CPU: 3 PID: 469 Comm: hackbench Not tainted 4.20.0-00005-g96b18288a88e-dirty > | > | [ECR ]: 0x00000000 => Check Programmer's Manual > | [EFA ]: 0x00000000 > | [BLINK ]: do_exit+0x4a6/0x7d0 > | [ERET ]: _raw_write_unlock_irq+0x44/0x5c > > - And while at it, remove the extar smp_mb() from EX based > arch_read_trylock() since the spin lock there guarantees a full > barrier anyways > > - For LLSC case, hackbench threads improves with this patch (HAPS @ 50MHz) > > ---- before ---- > | > | [ARCLinux]# for i in 1 2 3 4 5; do hackbench 10 thread; done > | Running with 10 groups 400 threads > | Time: 16.253 > | Time: 16.445 > | Time: 16.590 > | Time: 16.721 > | Time: 16.544 > > ---- after ---- > | > | [ARCLinux]# for i in 1 2 3 4 5; do hackbench 10 thread; done > | Running with 10 groups 400 threads > | Time: 15.638 > | Time: 15.730 > | Time: 15.870 > | Time: 15.842 > | Time: 15.729 > > Signed-off-by: Vineet Gupta <vgupta@xxxxxxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> _______________________________________________ linux-snps-arc mailing list linux-snps-arc@xxxxxxxxxxxxxxxxxxx http://lists.infradead.org/mailman/listinfo/linux-snps-arc
