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Subject: Re: [PATCH] rcu: restore correct batch limiting
From: Eric Dumazet <eric.dumazet@gmail.com>
To: paulmck@linux.vnet.ibm.com
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Date: Thu, 18 Oct 2012 18:10:28 +0200
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On Thu, 2012-10-18 at 08:25 -0700, Paul E. McKenney wrote:
> On Thu, Oct 18, 2012 at 02:44:50PM +0200, Eric Dumazet wrote:

> > 
> > Having 2 billions callbacks on one cpu would be problematic, I really
> > hope nobody relies on this ;)
> 
> Fair point!  ;-)
> 
> But just making everything long makes it quite easy to analyze.
> 
> > I guess the 10/infinity switch should be smarter.
> > 
> > something like the following maybe :
> > 
> > rdp->blimit = max(blimit, rdp->qlen >> 6);
> > 
> > (if queue is big, dont wait to hit 10000 before allowing more items to
> > be handled per round)
> 
> The -rt guys would not be amused.  :-(
> 
> But for non-realtime use, increasing rcutree.blimit either at boot or
> via sysfs could make sense.  It is also likely that I will move callback
> processing to a kthread at some point, which would allow some additional
> flexibility.
> 

Ah, I now realize the loop can exceed blimit, but is it true for BH
variant ? (Not really a problem for 3.6/3.7 kernels, but prior ones)

if (++count >= bl &&
    (need_resched() ||
     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
        break;

I wonder if ksoftirqd should be included as well...

> Furthermore, it would be easy to have one default for non-rt and another
> for -rt, if that would help.
> 
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > 
> > Please dont forget stable teams.  (3.2 + )
> 
> Added both, please see below!
> 

Seems fine to me, thanks Paul !