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SubjectRe: [PATCH 2/2] rcu: Force resched_cpu when jiffies >= rcu_state.jiffies_resched
On Mon, Nov 28, 2016 at 03:46:39PM +0900, Byungchul Park wrote:
> On Sat, Nov 26, 2016 at 04:00:46AM -0800, Paul E. McKenney wrote:
> > 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?
>
> Hello,
>
> I see.. I didn't check the log 'rcu: Stop treating in-kernel CPU-bound
> workloads as errors'.
>
> If you are convinced to remove them, no objection.

It is quite possible that additional time-related adjustments are needed,
but this change is a step in the right direction.

Thanx, Paul

> Thank you,
> Byungchul
>
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit d2db185bfee894c573faebed93461e9938bdbb61
> > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 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@lge.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 5f4f80d07852..91a68e4e6671 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -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. */
> > -
> > return 0;
> > }
> >
>

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