linux-kernel - Re: [PATCH 1/3] tracing/stat: sort in ascending order

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Date:	Tue, 26 May 2009 22:46:49 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] tracing/stat: sort in ascending order

On Tue, May 26, 2009 at 09:09:24AM +0800, Li Zefan wrote:
> Frederic Weisbecker wrote:
> > On Mon, May 25, 2009 at 04:46:09PM +0800, Li Zefan wrote:
> >> Currently the output of trace_stat/workqueues is totally reversed:
> >>
> >>  # cat /debug/tracing/trace_stat/workqueues
> >>     ...
> >>     1       17       17      210       37   `-blk_unplug_work+0x0/0x57
> >>     1     3779     3779      181       11   |-cfq_kick_queue+0x0/0x2f
> >>     1     3796     3796                     kblockd/1:120
> >>     ...
> >>
> >> The correct output should be:
> >>
> >>     1     3796     3796                     kblockd/1:120
> >>     1     3779     3779      181       11   |-cfq_kick_queue+0x0/0x2f
> >>     1       17       17      210       37   `-blk_unplug_work+0x0/0x57
> >>
> >> It's caused by "tracing/stat: replace linked list by an rbtree for sorting"
> >> (53059c9b67a62a3dc8c80204d3da42b9267ea5a0).
> >>
> >> Though we can simply change dummy_cmp() to return -1 instead of 1, IMO
> >> it's better to always do ascending sorting in trace_stat.c, and leave each
> >> stat tracer to decide whether to sort in descending or ascending order.
> >>
> >> [ Impact: fix the output of trace_stat/workqueue ]
> >>
> >> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> > 
> > 
> > For now in stat tracing, the ascendent sorting is the most relevant.
> > Especially because we always want to see the highest problems first.
> > 
> 
> Yeah, I saw this.
> 
> > -1 (or < 0) usually means lower and 1 ( > 0) is higher.
> > 
> > I wonder what would most confuse the developers of stat tracers:
> > 
> > - to reverse these common sort values (-1 turn into "higher")
> > - keep the default ascendent sorting, which is not natural because the default
> >   is often descendent.
> > 
> > I don't know. 
> > 
> > Anyone else. Do you have a preference?
> > 
> 
> When I looked into this bug, I was confused why it's descending sorting, though
> then I found out the answer.
> 
> Imagine a new stat tracer wants to sort in ascending order, but it has to define
> a cmp which compares in reverse order. This seems to be odd and confusing.
> 
> But to be honest, I'm also not sure which is better, And it doesn't seem to be
> a big issue. So I think I'll make concessions.


Ok, so let's keep it as is and we'll see if developers complain :)

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