6.1-stable review patch. If anyone has any objections, please let me know. ------------------ From: Yicong Yang <yangyicong@xxxxxxxxxxxxx> [ Upstream commit 54624acf8843375a6de3717ac18df3b5104c39c5 ] The test thread will start N benchmark kthreads and then schedule out until the test time finished and notify the benchmark kthreads to stop. The benchmark kthreads will keep running until notified to stop. There's a problem with current implementation when the benchmark kthreads number is equal to the CPUs on a non-preemptible kernel: since the scheduler will balance the kthreads across the CPUs and when the test time's out the test thread won't get a chance to be scheduled on any CPU then cannot notify the benchmark kthreads to stop. This can be easily reproduced on a VM (simulated with 16 CPUs) with PREEMPT_VOLUNTARY: estuary:/mnt$ ./dma_map_benchmark -t 16 -s 1 rcu: INFO: rcu_sched self-detected stall on CPU rcu: 10-...!: (5221 ticks this GP) idle=ed24/1/0x4000000000000000 softirq=142/142 fqs=0 rcu: (t=5254 jiffies g=-559 q=45 ncpus=16) rcu: rcu_sched kthread starved for 5255 jiffies! g-559 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=12 rcu: Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. rcu: RCU grace-period kthread stack dump: task:rcu_sched state:R running task stack:0 pid:16 tgid:16 ppid:2 flags:0x00000008 Call trace __switch_to+0xec/0x138 __schedule+0x2f8/0x1080 schedule+0x30/0x130 schedule_timeout+0xa0/0x188 rcu_gp_fqs_loop+0x128/0x528 rcu_gp_kthread+0x1c8/0x208 kthread+0xec/0xf8 ret_from_fork+0x10/0x20 Sending NMI from CPU 10 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 332 Comm: dma-map-benchma Not tainted 6.10.0-rc1-vanilla-LSE #8 Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015 pstate: 20400005 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : arm_smmu_cmdq_issue_cmdlist+0x218/0x730 lr : arm_smmu_cmdq_issue_cmdlist+0x488/0x730 sp : ffff80008748b630 x29: ffff80008748b630 x28: 0000000000000000 x27: ffff80008748b780 x26: 0000000000000000 x25: 000000000000bc70 x24: 000000000001bc70 x23: ffff0000c12af080 x22: 0000000000010000 x21: 000000000000ffff x20: ffff80008748b700 x19: ffff0000c12af0c0 x18: 0000000000010000 x17: 0000000000000001 x16: 0000000000000040 x15: ffffffffffffffff x14: 0001ffffffffffff x13: 000000000000ffff x12: 00000000000002f1 x11: 000000000001ffff x10: 0000000000000031 x9 : ffff800080b6b0b8 x8 : ffff0000c2a48000 x7 : 000000000001bc71 x6 : 0001800000000000 x5 : 00000000000002f1 x4 : 01ffffffffffffff x3 : 000000000009aaf1 x2 : 0000000000000018 x1 : 000000000000000f x0 : ffff0000c12af18c Call trace: arm_smmu_cmdq_issue_cmdlist+0x218/0x730 __arm_smmu_tlb_inv_range+0xe0/0x1a8 arm_smmu_iotlb_sync+0xc0/0x128 __iommu_dma_unmap+0x248/0x320 iommu_dma_unmap_page+0x5c/0xe8 dma_unmap_page_attrs+0x38/0x1d0 map_benchmark_thread+0x118/0x2c0 kthread+0xec/0xf8 ret_from_fork+0x10/0x20 Solve this by adding scheduling point in the kthread loop, so if there're other threads in the system they may have a chance to run, especially the thread to notify the test end. However this may degrade the test concurrency so it's recommended to run this on an idle system. Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx> Acked-by: Barry Song <baohua@xxxxxxxxxx> Signed-off-by: Christoph Hellwig <hch@xxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- kernel/dma/map_benchmark.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c index dafdc47ae5fc..3efdc5cfe390 100644 --- a/kernel/dma/map_benchmark.c +++ b/kernel/dma/map_benchmark.c @@ -89,6 +89,22 @@ static int map_benchmark_thread(void *data) atomic64_add(map_sq, &map->sum_sq_map); atomic64_add(unmap_sq, &map->sum_sq_unmap); atomic64_inc(&map->loops); + + /* + * We may test for a long time so periodically check whether + * we need to schedule to avoid starving the others. Otherwise + * we may hangup the kernel in a non-preemptible kernel when + * the test kthreads number >= CPU number, the test kthreads + * will run endless on every CPU since the thread resposible + * for notifying the kthread stop (in do_map_benchmark()) + * could not be scheduled. + * + * Note this may degrade the test concurrency since the test + * threads may need to share the CPU time with other load + * in the system. So it's recommended to run this benchmark + * on an idle system. + */ + cond_resched(); } out: -- 2.43.0
