linux-kernel - Re: [PATCH 0/2] ftrace: updates to tip

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Date:	Mon, 19 Jan 2009 14:31:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 0/2] ftrace: updates to tip


* Arnaldo Carvalho de Melo <acme@...hat.com> wrote:

> Em Fri, Jan 16, 2009 at 11:26:00PM +0100, Ingo Molnar escreveu:
> > 
> > * Chris Mason <chris.mason@...cle.com> wrote:
> > 
> > > On Thu, 2009-01-15 at 19:40 -0500, Steven Rostedt wrote:
> > > > Ingo,
> > > > 
> > > > The first patch is critical, and needs to stay with trace_output.c
> > > > Not that critical since trace_output.c is not in mainline yet.
> > > > 
> > > > The second patch gives the ability to stack trace functions.
> > > > I've been leery about adding this and still keep it a separate
> > > > option from the "stacktrace" that already exists. This is because
> > > > when enabled with no filtering, the lag between typing and seeing
> > > > what is typed can be up to 10 seconds or more.
> > > > 
> > > 
> > > I mostly asked for this because I often try to find the most common
> > > reason a given function is called, and oprofile isn't always a great way
> > > to catch it.  systemtap can do it too if you can get systemtap to work
> > > against your current devel kernel, but there are limits on how much
> > > memory it'll use.
> > > 
> > > I've attached some simple python code that parses the output of the
> > > function stack tracer, it makes some dumb assumptions about the format
> > > but isn't a bad proof of concept.  The first such assumption is that
> > > you're only filtering on a single function.
> > > 
> > > Here is some example output, trying to find the most common causes of
> > > native_smp_send_reschedule() during a btrfs dbench run.
> > > 
> > > It relates to the Oracle OLTP thread because oracle heavily uses IPC
> > > semaphores to trigger wakeups of processes as various events finish.
> > > I'd bet that try_to_wakeup is the most common cause of the reschedule
> > > calls there as well.
> > > 
> > > For btrfs, the btree lock mutexes come back into the profile yet again.
> > > It would be interesting to change the spinning mutex code to look for
> > > spinners and skip the wakeup on unlock, but that's a different thread
> > > entirely.
> > > 
> > > The short version is: thanks Steve, this is really cool!
> > > 
> > > 12058 hits: 
> > >  <= check_preempt_wakeup
> > >  <= try_to_wake_up
> > >  <= wake_up_process
> > >  <= __mutex_unlock_slowpath
> > >  <= mutex_unlock
> > >  <= btrfs_tree_unlock
> > >  <= unlock_up
> > >  ===========
> > 
> > Cool! We've got scripts/tracing/ [with one Python script in it already] - 
> > so if this is tidied up to be generally useful we could put it there.
> > 
> > The other thing is that there's the statistics framework of ftrace, being 
> > worked on by Frederic and Steve. That tries to handle and provide 
> > higher-order summaries/"views" of plain traces, like histograms and counts 
> > - provided by the kernel.
> 
> That I plan to use to provide something similar (equal?) in blkftrace to what
> blktrace produces when it stops a tracing session:
> 
>   8,0    1     3767     2.180288230 31765  C   W 126227253 + 8 [0]
> ^CCPU0 (8,0):
>  Reads Queued:           0,        0KiB	 Writes Queued:         141, 564KiB
>  Read Dispatches:        0,        0KiB	 Write Dispatches:        0, 0KiB
>  Reads Requeued:         0		 Writes Requeued:         0
>  Reads Completed:        0,        0KiB	 Writes Completed:        0, 0KiB
>  Read Merges:            0,        0KiB	 Write Merges:           45, 180KiB
>  Read depth:             1        	 Write depth:             2
>  IO unplugs:             2        	 Timer unplugs:           0
> CPU1 (8,0):
>  Reads Queued:           2,        8KiB	 Writes Queued:         679, 2,716KiB
>  Read Dispatches:        2,        8KiB	 Write Dispatches:      598, 3,280KiB
>  Reads Requeued:         0		 Writes Requeued:         0
>  Reads Completed:        2,        8KiB	 Writes Completed:      598, 3,280KiB
>  Read Merges:            0,        0KiB	 Write Merges:          177, 708KiB
>  Read depth:             1        	 Write depth:             2
>  IO unplugs:             3        	 Timer unplugs:           0
> 
> Total (8,0):
>  Reads Queued:           2,        8KiB	 Writes Queued:         820, 3,280KiB
>  Read Dispatches:        2,        8KiB	 Write Dispatches:      598, 3,280KiB
>  Reads Requeued:         0		 Writes Requeued:         0
>  Reads Completed:        2,        8KiB	 Writes Completed:      598, 3,280KiB
>  Read Merges:            0,        0KiB	 Write Merges:          222, 888KiB
>  IO unplugs:             5        	 Timer unplugs:           0
> 
> Throughput (R/W): 3KiB/s / 1,504KiB/s
> Events (8,0): 4,289 entries
> Skips: 0 forward (0 -   0.0%)
> [root@...pio ~]# 
> 
> :-)

Cool :)

	Ingo
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