Re: Slow pty's (was Re: libdivecomputer interfaces?)

[Mike Galbraith <mgalbraith@suse.de>]

On Wed, 2010-06-16 at 17:16 +0200, Mike Galbraith wrote:
> On Wed, 2010-06-16 at 17:03 +0200, Jiri Kosina wrote:
> > On Thu, 10 Jun 2010, Brian Bloniarz wrote:
> > 
> > > On 06/10/2010 02:10 PM, Chris Wedgwood wrote:
> > > > (sorry if this reponse isn't on target, i was just pointed to this
> > > > thread a few minutes ago)
> > > > 
> > > > 
> > > > On Thu, Jun 10, 2010 at 10:25:36AM -0700, Linus Torvalds wrote:
> > > > 
> > > >>  I thought we long since (ie back last fall) fixed the latency
> > > >> problems with pty's, but there does seem to be something very fishy
> > > >> going on there still.
> > > > 
> > > > this might not be related, but i have slow serial ports with NOHZ that
> > > > goes away when i revert 39c0cbe2150cbd848a25ba6cdb271d1ad46818ad.
> > > 
> > > Unrelated or not, I think Chris is right about this. Somewhere before
> > > -rc1, the emulated serial console on my KVM instance became slow
> > > to echo input. I just tested with the commit reverted and it's
> > > back to normal.
> > 
> > So let's CC Mike then.
> 
> Chris already gave me a heads up, it's on my todo.  The old P4 box I use
> for a serial console box is exploding on boot, or I would have already
> had a look.

(Removing filth and re-seating ram seems to have revived poor old P4)

I'm not seeing any problem with serial console here, seems to work just
fine P4->Q6600, both running NOHZ kernels with nohz_ratelimit(), 33.5 on
the P4, and tip.today on the Q6600.

Eyeballing it, perhaps we need to proceed downward if any needs_cpu
condition is true, despite having just been here a wee bit ago.

Does this help anyone's woes?

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 5f171f0..ec72fad 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -262,7 +262,7 @@ void tick_nohz_stop_sched_tick(int inidle)
 	ktime_t last_update, expires, now;
 	struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
 	u64 time_delta;
-	int cpu;
+	int cpu, cpu_needed;
 
 	local_irq_save(flags);
 
@@ -315,7 +315,9 @@ void tick_nohz_stop_sched_tick(int inidle)
 		goto end;
 	}
 
-	if (nohz_ratelimit(cpu))
+	cpu_needed = rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) || arch_needs_cpu(cpu);
+
+	if (!cpu_needed && nohz_ratelimit(cpu))
 		goto end;
 
 	ts->idle_calls++;
@@ -327,8 +329,7 @@ void tick_nohz_stop_sched_tick(int inidle)
 		time_delta = timekeeping_max_deferment();
 	} while (read_seqretry(&xtime_lock, seq));
 
-	if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
-	    arch_needs_cpu(cpu)) {
+	if (cpu_needed) {
 		next_jiffies = last_jiffies + 1;
 		delta_jiffies = 1;
 	} else {