linux-kernel - Re: [PATCH 2/2] rcu: Force resched_cpu when jiffies >= rcu_state.jiffies

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Date:   Mon, 28 Nov 2016 15:46:39 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rcu: Force resched_cpu when jiffies >=
 rcu_state.jiffies_resched

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.

Thank you,
Byungchul

> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit d2db185bfee894c573faebed93461e9938bdbb61
> Author: Paul E. McKenney <paulmck@...ux.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@....com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.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;
>  }
>