linux-kernel - Re: [patch] fix uml slowness caused by ptrace preemption bug on host

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Date:	Fri, 20 Mar 2009 09:27:10 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	peterz@...radead.org
CC:	miklos@...redi.hu, roland@...hat.com, efault@....de, rjw@...k.pl,
	jdike@...toit.com, mingo@...e.hu,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] fix uml slowness caused by ptrace preemption bug on
 host

On Fri, 20 Mar 2009, Peter Zijlstra wrote:
> On Thu, 2009-03-19 at 23:23 +0100, Miklos Szeredi wrote:
> > 
> > This patch solves this by not scheduling on preempt_enable() after
> > ptrace_stop() has woken up the tracer.
> 
> Nice,.. however did you find this?

Ftrace helped a lot, it's a really cool tool :).  I had to patch it
with this, otherwise the timestamps would be totally off:

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index bd38c5c..557c2dd 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -108,7 +108,7 @@ u64 ring_buffer_time_stamp(int cpu)
 
 	preempt_disable_notrace();
 	/* shift to debug/test normalization and TIME_EXTENTS */
-	time = sched_clock() << DEBUG_SHIFT;
+	time = cpu_clock(cpu) << DEBUG_SHIFT;
 	preempt_enable_no_resched_notrace();
 
 	return time;
--
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