From linux-kernel  Thu Jun 17 14:11:59 2010
From: Thomas Gleixner <tglx () linutronix ! de>
Date: Thu, 17 Jun 2010 14:11:59 +0000
To: linux-kernel
Subject: Re: Slow pty's (was Re: libdivecomputer interfaces?)
Message-Id: <alpine.LFD.2.00.1006171606170.4290 () localhost ! localdomain>
X-MARC-Message: https://marc.info/?l=linux-kernel&m=127678398531381

On Thu, 17 Jun 2010, Peter Zijlstra wrote:

> On Thu, 2010-06-17 at 12:50 +0200, Mike Galbraith wrote:
> 
> > sched: do not ratelimit NOHZ when the tick is stopped.
> > 
> > Chris Wedgwood reports that 39c0cbe sched: Rate-limit nohz causes a serial
> > console regression, unresponsiveness, and indeed it does.  The below fixes
> > it by not skipping out when the tick has been stopped.
> > 
> > Tested that the throughput benefit of ratelimiting is still intact.  It is.
> > 
> > Signed-off-by: Mike Galbraith <efault@gmx.de>
> > Reported-by: Chris Wedgwood <cw@f00f.org>
> 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 5f171f0..83c5129 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -315,7 +315,7 @@ void tick_nohz_stop_sched_tick(int inidle)
> >  		goto end;
> >  	}
> >  
> > -	if (nohz_ratelimit(cpu))
> > +	if (!ts->tick_stopped && nohz_ratelimit(cpu))
> >  		goto end;
> >  
> >  	ts->idle_calls++;
> >  
> 
> 
> Humm,. the code around there suggests something like the below, but I
> must admit its been a while since I really read all that nohz stuff,
> Thomas, any preferences?

The version below is better as it solves the problem and follows the
nohz_ratelimit() advise even in the case where it changes after the
tick has been stopped.

Thanks,

	tglx
> ---
>  kernel/time/tick-sched.c |    5 +----
>  1 files changed, 1 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 5f171f0..e0707ea 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -315,9 +315,6 @@ void tick_nohz_stop_sched_tick(int inidle)
>  		goto end;
>  	}
>  
> -	if (nohz_ratelimit(cpu))
> -		goto end;
> -
>  	ts->idle_calls++;
>  	/* Read jiffies and the time when jiffies were updated last */
>  	do {
> @@ -328,7 +325,7 @@ void tick_nohz_stop_sched_tick(int inidle)
>  	} while (read_seqretry(&xtime_lock, seq));
>  
>  	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
> -	    arch_needs_cpu(cpu)) {
> +	    arch_needs_cpu(cpu) || nohz_ratelimit(cpu)) {
>  		next_jiffies = last_jiffies + 1;
>  		delta_jiffies = 1;
>  	} else {
> 
> 
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