This is a note to let you know that I've just added the patch titled
dma-mapping: benchmark: Don't starve others when doing the test
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
dma-mapping-benchmark-don-t-starve-others-when-doing.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 90ffa65d38d556a0631baa519689dacdfe1dcae3
Author: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
Date: Thu Jun 20 17:28:55 2024 +0800
dma-mapping: benchmark: Don't starve others when doing the test
[ 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>
diff --git a/kernel/dma/map_benchmark.c b/kernel/dma/map_benchmark.c
index 4950e0b622b1..cc19a3efea89 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:
