From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Anton Blanchard <anton@samba.org>
Cc: linuxppc-dev@ozlabs.org, paulus@samba.org, imunsie@au1.ibm.com,
rostedt@goodmis.org, amodra@gmail.com
Subject: Re: PowerPC ftrace function trace optimisation
Date: Thu, 29 Apr 2010 11:08:14 +1000 [thread overview]
Message-ID: <1272503294.24542.140.camel@pasglop> (raw)
In-Reply-To: <1272502967.24542.137.camel@pasglop>
On Thu, 2010-04-29 at 11:02 +1000, Benjamin Herrenschmidt wrote:
> > The option Alan added reduces the footprint to 3 instructions which can
> > be noped out completely. The rest of the function does not rely on the first
> > three instructions. No stack spill is forced either:
> >
> > # gcc -pg -mprofile-kernel
>
> >From a quick test it appears that this only works with -m64, not -m32.
> Alan is that correct ? Any chance you can fix that in future gcc
> versions ?
>
> Also should we implement support for both type of mcounts or just only
> allow enabling of ftrace with gcc's that support this ?
Also, Anton noticed :
> Cheers,
> Ben.
>
> > 0000000000000000 <.foo>:
> > 0: 7c 08 02 a6 mflr r0
> > 4: f8 01 00 10 std r0,16(r1)
The std is not useful here. We can do it inside mcount.
> > 8: 48 00 00 01 bl 8 <.foo+0x8> <--- call to mcount
And I noticed:
> > c: 7c 08 02 a6 mflr r0
I'm happy to guarantee that mcount does the above.
> > 10: f8 01 00 10 std r0,16(r1)
And maybe that one too.
However I understand if it's easier not to change the prolog codegen
(the 2 insn above) and just stick to adding a 2 or 3 instructions
boilerplate at the top.
Cheers,
Ben.
> > 14: f8 21 ff d1 stdu r1,-48(r1)
> > 18: e9 22 00 00 ld r9,0(r2)
> > 1c: e8 69 00 02 lwa r3,0(r9)
> > 20: 38 21 00 30 addi r1,r1,48
> > 24: e8 01 00 10 ld r0,16(r1)
> > 28: 7c 08 03 a6 mtlr r0
> > 2c: 4e 80 00 20 blr
> >
> >
> > This mean we could support ftrace function trace with very little overhead.
> >
> > In fact if we are careful when switching to the new mcount ABI and don't
> > rely on the store of r0, we could probably optimise this even further in a
> > future gcc and remove the store completely. mcount would be 2 instructions:
> >
> > mflr r0
> > bl 8 <.foo+0x8>
> >
> > Anton
>
next prev parent reply other threads:[~2010-04-29 1:08 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-04-29 0:51 PowerPC ftrace function trace optimisation Anton Blanchard
2010-04-29 1:02 ` Benjamin Herrenschmidt
2010-04-29 1:08 ` Benjamin Herrenschmidt [this message]
2010-04-29 1:22 ` Alan Modra
2010-04-29 1:55 ` Steven Rostedt
2010-04-29 2:10 ` Benjamin Herrenschmidt
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