Messages in this thread | | | Date | Fri, 25 May 2018 13:11:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v4] rcu: Speed up calling of RCU tasks callbacks |
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On Thu, May 24, 2018 at 06:49:46PM -0400, Steven Rostedt wrote: > > From: Steven Rostedt (VMware) <rostedt@goodmis.org> > > 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-joel@joelfernandes.org > Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Suggested-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
I queued both commits, thank you all!
Thanx, Paul
> --- > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > index 68fa19a5e7bd..452e47841a86 100644 > --- a/kernel/rcu/update.c > +++ b/kernel/rcu/update.c > @@ -715,6 +715,7 @@ 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); > @@ -796,13 +797,25 @@ 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); >
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