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authorShaohua Li <shaohua.li@intel.com>2011-06-14 01:26:25 -0400
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2011-06-14 18:25:39 -0400
commit09223371deac67d08ca0b70bd18787920284c967 (patch)
tree0ad0354a93ac209c8cfd2b79d03d0462aa3c5c58 /Documentation
parent9a432736904d386cda28b987b38ba14dae960ecc (diff)
rcu: Use softirq to address performance regression
Commit a26ac2455ffcf3(rcu: move TREE_RCU from softirq to kthread) introduced performance regression. In an AIM7 test, this commit degraded performance by about 40%. The commit runs rcu callbacks in a kthread instead of softirq. We observed high rate of context switch which is caused by this. Out test system has 64 CPUs and HZ is 1000, so we saw more than 64k context switch per second which is caused by RCU's per-CPU kthread. A trace showed that most of the time the RCU per-CPU kthread doesn't actually handle any callbacks, but instead just does a very small amount of work handling grace periods. This means that RCU's per-CPU kthreads are making the scheduler do quite a bit of work in order to allow a very small amount of RCU-related processing to be done. Alex Shi's analysis determined that this slowdown is due to lock contention within the scheduler. Unfortunately, as Peter Zijlstra points out, the scheduler's real-time semantics require global action, which means that this contention is inherent in real-time scheduling. (Yes, perhaps someone will come up with a workaround -- otherwise, -rt is not going to do well on large SMP systems -- but this patch will work around this issue in the meantime. And "the meantime" might well be forever.) This patch therefore re-introduces softirq processing to RCU, but only for core RCU work. RCU callbacks are still executed in kthread context, so that only a small amount of RCU work runs in softirq context in the common case. This should minimize ksoftirqd execution, allowing us to skip boosting of ksoftirqd for CONFIG_RCU_BOOST=y kernels. Signed-off-by: Shaohua Li <shaohua.li@intel.com> Tested-by: "Alex,Shi" <alex.shi@intel.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/proc.txt1
1 files changed, 1 insertions, 0 deletions
diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index f48178024067..db3b1aba32a3 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -843,6 +843,7 @@ Provides counts of softirq handlers serviced since boot time, for each cpu.
843 TASKLET: 0 0 0 290 843 TASKLET: 0 0 0 290
844 SCHED: 27035 26983 26971 26746 844 SCHED: 27035 26983 26971 26746
845 HRTIMER: 0 0 0 0 845 HRTIMER: 0 0 0 0
846 RCU: 1678 1769 2178 2250
846 847
847 848
8481.3 IDE devices in /proc/ide 8491.3 IDE devices in /proc/ide