Commit-ID: c03be752d39dc64dcfda0ac8ce87fb10b1ee5621 Gitweb: https://git.kernel.org/tip/c03be752d39dc64dcfda0ac8ce87fb10b1ee5621 Author: Steven Rostedt (VMware) <rost...@goodmis.org> AuthorDate: Thu, 24 May 2018 18:49:46 -0400 Committer: Paul E. McKenney <paul...@linux.vnet.ibm.com> CommitDate: Thu, 12 Jul 2018 15:39:21 -0700
rcu: Speed up calling of RCU tasks callbacks Joel Fernandes found that the synchronize_rcu_tasks() was taking a significant amount of time. He demonstrated it with the following test: # cd /sys/kernel/tracing # while [ 1 ]; do x=1; done & # echo '__schedule_bug:traceon' > set_ftrace_filter # time echo '!__schedule_bug:traceon' > set_ftrace_filter; real 0m1.064s user 0m0.000s sys 0m0.004s Where it takes a little over a second to perform the synchronize, because there's a loop that waits 1 second at a time for tasks to get through their quiescent points when there's a task that must be waited for. After discussion we came up with a simple way to wait for holdouts but increase the time for each iteration of the loop but no more than a full second. With the new patch we have: # time echo '!__schedule_bug:traceon' > set_ftrace_filter; real 0m0.131s user 0m0.000s sys 0m0.004s Which drops it down to 13% of what the original wait time was. Link: http://lkml.kernel.org/r/20180523063815.198302-2-j...@joelfernandes.org Reported-by: Joel Fernandes (Google) <j...@joelfernandes.org> Suggested-by: Joel Fernandes (Google) <j...@joelfernandes.org> Signed-off-by: Steven Rostedt (VMware) <rost...@goodmis.org> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> --- kernel/rcu/update.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 5783bdf86e5a..4c7c49c106ee 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -668,6 +668,7 @@ static int __noreturn rcu_tasks_kthread(void *arg) struct rcu_head *list; struct rcu_head *next; LIST_HEAD(rcu_tasks_holdouts); + int fract; /* Run on housekeeping CPUs by default. Sysadm can move if desired. */ housekeeping_affine(current, HK_FLAG_RCU); @@ -749,13 +750,25 @@ static int __noreturn rcu_tasks_kthread(void *arg) * holdouts. When the list is empty, we are done. */ lastreport = jiffies; - while (!list_empty(&rcu_tasks_holdouts)) { + + /* Start off with HZ/10 wait and slowly back off to 1 HZ wait*/ + fract = 10; + + for (;;) { bool firstreport; bool needreport; int rtst; struct task_struct *t1; - schedule_timeout_interruptible(HZ); + if (list_empty(&rcu_tasks_holdouts)) + break; + + /* Slowly back off waiting for holdouts */ + schedule_timeout_interruptible(HZ/fract); + + if (fract > 1) + fract--; + rtst = READ_ONCE(rcu_task_stall_timeout); needreport = rtst > 0 && time_after(jiffies, lastreport + rtst);
