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

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Message-ID: <20090320083049.GA29471@elte.hu>
Date:	Fri, 20 Mar 2009 09:30:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	peterz@...radead.org, roland@...hat.com, efault@....de,
	rjw@...k.pl, jdike@...toit.com,
	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


* Miklos Szeredi <miklos@...redi.hu> wrote:

> 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();

Btw., based on your earlier report, the same is now possible in the 
latest tracing tree via:

   echo 1 > /debug/tracing/options/global_clock

	Ingo
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