Date:	Fri, 11 Jan 2008 14:21:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	David Dillow <dillowda@...l.gov>,
	Guillaume Chazarain <guichaz@...oo.fr>,
	linux-kernel@...r.kernel.org, linux-btrace@...r.kernel.org,
	mingo@...hat.com, tglx@...utronix.de
Subject: Re: [patch] block: fix blktrace timestamps


* Jens Axboe <jens.axboe@...cle.com> wrote:

> > > If blktrace worked in 2.6.23 and it doesn't in 2.6.24 because of 
> > > some option that isn't immediately apparent, then it's a 
> > > regression. Period.
> > 
> > not completely correct. CONFIG_NO_HZ is a default-disabled option 
> > that became newly available on 64-bit x86. So if NO_HZ does not 
> > completely work on 64-bit, and if 32-bit works fine - which we dont 
> > know yet (my guess would be that it's similarly broken on the same 
> > box) then it's not a regression.
> 
> Ingo, it doesn't matter if the option is disabled by default or not! 
> The fact is that functionality foo works in 2.6.23 and doesn't in 
> 2.6.24 because of something unrelated. And that IS a regression, no 
> matter what kind of word play you are doing here :-)

argh, Jens. Really. I believe you stopped using your brain because 
CONFIG_BKL_IO_TRACE=y is affected by this bug and apparently you've got 
a weak spot for it ;)

Think about it another way then, in the context of another, real kernel 
feature we introduced in v2.6.24, namely CONFIG_CPU_IDLE=y:

- Fact: feature CONFIG_CPU_IDLE=y did not exist in 64-bit x86 in v2.6.23.
  It has known bugs but they are not flagged 'regressions' (because the 
  feature did not exist before and the option is default-disabled) hence 
  the feature can stay. Some fixes to it are too dangerous or too 
  unproven and are delayed to v2.6.25.

and now apply the same rule to CONFIG_NO_HZ:

- Fact: feature CONFIG_NO_HZ=y did not exist in 64-bit x86 in v2.6.23.
  It has known bugs but they are not flagged 'regressions' (because the 
  feature did not exist before and the option is default-disabled) hence 
  the feature can stay. Some fixes to it are too dangerous or too 
  unproven and are delayed to v2.6.25.

ok?

Yes, it's bad that this happened, and perhaps it _is_ a regression, but 
not for the reason you claim. [ It might be a regression if 32-bit 
blktrace has the same problem under NO_HZ for example _AND_ bkltrace 
worked perfectly on the same box on v2.6.23. ]

Kernel regressions have a _strict_ definition: they mean "anything that 
worked before will work in the future too". Not: "anything that worked 
before will work in the future too if you enable random new non-default 
kernel features".

[ If the latter was the criterium we'd probably never see new features
  integrated! New stuff has bugs, because it's not nearly as well-tested 
  as older stuff. What matters is to 1) not turn it on by default if it 
  has known bugs 2) to always make positive progress on it, i.e.: to not
  regress new features in future kernel releases. This way, in the ideal 
  case, we'll have a monotonic series towards a perfect, bug-free kernel 
  ;) ]

> > ktime_get() should have been used instead, which is a proper GTOD 
> > clocksource. The patch below implements this.
> 
> Will give it a whirl, it looks promising indeed and gets rid of the 
> ugly cpu sync stuff. [...]

fantastic! :)

> [...] What is the cost of ktime_get() compared to sched_clock()?

compared to the costs of IO transfers it should be OK. It can be really 
fast but in the worst-case it can be as slow as 3-6 usecs (when we use 
the acpi_pm clocksource). If there's complaints then probably the 
networking folks will yell first :)

	Ingo
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