Re: [PATCH 2/2] rcu: Force resched_cpu when jiffies >= rcu_state.jiffies_resched
From: Paul E. McKenney
Date: Sat Nov 26 2016 - 15:42:49 EST
On Fri, Nov 25, 2016 at 12:34:26AM -0800, Paul E. McKenney wrote:
> On Tue, Nov 22, 2016 at 05:12:20PM +0900, Byungchul Park wrote:
> > On Wed, Nov 09, 2016 at 03:32:15PM +0900, Byungchul Park wrote:
> > > Currently rcu code forces CPU into scheduler when jiffies >=
> > > rcu_state.gp_start + jiffies_till_sched_qs, via resched_cpu().
> > >
> > > It would be better to force CPU into scheduler when jiffies >=
> > > rcu_state.jiffies_resched, too.
> > Hello,
> > I think these two patches are necessary to call resched_cpu() even in
> > case of jiffies >= rcu_state.jiffies_resched, too. Am I wrong?
> > It would be appriciated if you let me know if I was wrong.
> My current thought is that both the "if" statement and the call to
> resched_cpu() should be removed, but I am still testing and working
> through the timing.
> Either way, I do very much appreciate your having called my attention
> to this code!
And the patch shown below seems to do the trick. Thoughts?
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Fri Nov 25 00:07:23 2016 -0800
rcu: Remove short-term CPU kicking
Commit 4914950aaa12d ("rcu: Stop treating in-kernel CPU-bound workloads
as errors") added a (relatively) short-timeout call to resched_cpu().
This was inspired by as issue that was fixed by b7e7ade34e61 ("sched/core:
Fix remote wakeups"). But given that this issue was fixed, it is time
for the current commit to remove this call to resched_cpu().
Reported-by: Byungchul Park <byungchul.park@xxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5f4f80d07852..91a68e4e6671 100644
@@ -1345,12 +1345,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
- /* And if it has been a really long time, kick the CPU as well. */
- if (ULONG_CMP_GE(jiffies,
- rdp->rsp->gp_start + 2 * jiffies_till_sched_qs) ||
- ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + jiffies_till_sched_qs))
- resched_cpu(rdp->cpu); /* Force CPU into scheduler. */