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From: Steven Rostedt <rostedt@goodmis.org>
To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Christoph Hellwig <hch@infradead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
	Gregory Haskins <ghaskins@novell.com>,
	Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tim Bird <tim.bird@am.sony.com>, Sam Ravnborg <sam@ravnborg.org>,
	"Frank Ch. Eigler" <fche@redhat.com>,
	John Stultz <johnstul@us.ibm.com>,
	Arjan van de Ven <arjan@infradead.org>,
	Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation
Date: Wed, 30 Jan 2008 08:53:24 -0500 (EST)	[thread overview]
Message-ID: <Pine.LNX.4.58.0801300845180.29020@gandalf.stny.rr.com> (raw)
In-Reply-To: <47A079BD.8020106@siemens.com>


On Wed, 30 Jan 2008, Jan Kiszka wrote:

> Steven Rostedt wrote:
>
> > --- linux-mcount.git.orig/arch/x86/kernel/entry_32.S	2008-01-29 16:59:15.000000000 -0500
> > +++ linux-mcount.git/arch/x86/kernel/entry_32.S	2008-01-29 17:26:18.000000000 -0500
> > @@ -75,6 +75,31 @@ DF_MASK		= 0x00000400
> >  NT_MASK		= 0x00004000
> >  VM_MASK		= 0x00020000
> >
> > +#ifdef CONFIG_MCOUNT
> > +.globl mcount
> > +mcount:
> > +	/* unlikely(mcount_enabled) */
> > +	cmpl $0, mcount_enabled
> > +	jnz trace
> > +	ret
>
> (and the corresponding 64-bit version)
>
> Is the impact of this change on the (already expensive) mcount_enabled
> case negligible? I worried about use cases where we want to gain some
> (relative) worst-case numbers via these instrumentations.

The goal here was to limit the instruction cache hit that we take when
mcount_enabled = 0.
>
> In my personal priority scheme, CONFIG_MCOUNT=y && !mcount_enabled comes
> after mcount_enabled.

well, actually, I disagree. I only set mcount_enabled=1 when I'm about to
test something. You're right that we want the impact of the test least
affected, but when we have mcount_enabled=1 we usually also have a
function that's attached and in that case this change is negligible. But
on the normal case where mcount_enabled=0, this change may have a bigger
impact.

Remember CONFIG_MCOUNT=y && mcount_enabled=0 is (15% overhead)
         CONFIG_MCOUNT=y && mcount_enabled=1 dummy func (49% overhead)
         CONFIG_MCOUNT=y && mcount_enabled=1 trace func (500% overhead)

The trace func is the one that will be most likely used when analyzing. It
gives hackbench a 500% overhead, so I'm expecting this change to be
negligible in that case. But after I find what's wrong, I like to rebuild
the kernel without rebooting so I like to have mcount_enabled=0 have the
smallest impact ;-)

I'll put back the original code and run some new numbers.

-- Steve


  reply	other threads:[~2008-01-30 13:53 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-30  3:15 [PATCH 00/22 -v7] mcount and latency tracing utility -v7 Steven Rostedt
2008-01-30  3:15 ` [PATCH 01/22 -v7] printk - dont wakeup klogd with interrupts disabled Steven Rostedt
2008-01-30  3:15 ` [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation Steven Rostedt
2008-01-30  8:46   ` Peter Zijlstra
2008-01-30 13:08     ` Steven Rostedt
2008-01-30 14:09       ` Steven Rostedt
2008-01-30 14:25         ` Peter Zijlstra
2008-02-01 22:34           ` Paul E. McKenney
2008-02-02  1:56             ` Steven Rostedt
2008-02-02 21:41               ` Paul E. McKenney
2008-02-04 17:09                 ` Steven Rostedt
2008-02-04 21:40                   ` Paul E. McKenney
2008-02-04 22:03                     ` Steven Rostedt
2008-02-04 22:41                       ` Mathieu Desnoyers
2008-02-05  6:11                         ` Paul E. McKenney
2008-02-05  5:13                       ` Paul E. McKenney
2008-01-30 13:21   ` Jan Kiszka
2008-01-30 13:53     ` Steven Rostedt [this message]
2008-01-30 14:28       ` Steven Rostedt
2008-01-30  3:15 ` [PATCH 03/22 -v7] Annotate core code that should not be traced Steven Rostedt
2008-01-30  3:15 ` [PATCH 04/22 -v7] x86_64: notrace annotations Steven Rostedt
2008-01-30  3:15 ` [PATCH 05/22 -v7] add notrace annotations to vsyscall Steven Rostedt
2008-01-30  8:49   ` Peter Zijlstra
2008-01-30 13:15     ` Steven Rostedt
2008-01-30  3:15 ` [PATCH 06/22 -v7] handle accurate time keeping over long delays Steven Rostedt
2008-01-30  3:15 ` [PATCH 07/22 -v7] initialize the clock source to jiffies clock Steven Rostedt
2008-01-30  3:15 ` [PATCH 08/22 -v7] add get_monotonic_cycles Steven Rostedt
2008-01-30  3:15 ` [PATCH 09/22 -v7] add notrace annotations to timing events Steven Rostedt
2008-01-30  3:15 ` [PATCH 10/22 -v7] mcount based trace in the form of a header file library Steven Rostedt
2008-01-30  3:15 ` [PATCH 11/22 -v7] Add context switch marker to sched.c Steven Rostedt
2008-01-30  3:15 ` [PATCH 12/22 -v7] Make the task State char-string visible to all Steven Rostedt
2008-01-30  3:15 ` [PATCH 13/22 -v7] Add tracing of context switches Steven Rostedt
2008-01-30  3:15 ` [PATCH 14/22 -v7] Generic command line storage Steven Rostedt
2008-01-30  3:15 ` [PATCH 15/22 -v7] trace generic call to schedule switch Steven Rostedt
2008-01-30  3:15 ` [PATCH 16/22 -v7] Add marker in try_to_wake_up Steven Rostedt
2008-01-30  3:15 ` [PATCH 17/22 -v7] mcount tracer for wakeup latency timings Steven Rostedt
2008-01-30  9:31   ` Peter Zijlstra
2008-01-30 13:18     ` Steven Rostedt
2008-01-30  3:15 ` [PATCH 18/22 -v7] Trace irq disabled critical timings Steven Rostedt
2008-01-30  3:15 ` [PATCH 19/22 -v7] trace preempt off " Steven Rostedt
2008-01-30  9:40   ` Peter Zijlstra
2008-01-30 13:40     ` Steven Rostedt
2008-01-30  3:15 ` [PATCH 20/22 -v7] Add markers to various events Steven Rostedt
2008-01-30  3:15 ` [PATCH 21/22 -v7] Add event tracer Steven Rostedt
2008-01-30  3:15 ` [PATCH 22/22 -v7] Critical latency timings histogram Steven Rostedt

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