On Thu, Dec 6, 2012 at 5:36 PM, Liviu Dudau Liviu.Dudau@arm.com wrote:
On Thu, Dec 06, 2012 at 11:50:15AM +0000, Vincent Guittot wrote:
On 6 December 2012 11:27, Liviu Dudau Liviu.Dudau@arm.com wrote:
On Thu, Dec 06, 2012 at 09:32:11AM +0000, Viresh Kumar wrote:
Hi Vincent,
On 6 December 2012 14:50, Vincent Guittot vincent.guittot@linaro.org
wrote:
It's look like you have disabled packing small task in your v13
with 4a29297
It looks like you are referring to an older version of this branch as the commit id's don't match. Can you please check that again?
In addition, I don't fully understand why you absolutely want to revert 39b0e77. In the worst case, it will not fully solve what it should have solved but it doesn't add any regression.
I don't want to, but Liviu and Morten do :) @Liviu/Morten: Your comments
Hi Vincent,
I was just a messenger here, but this is my understanding of where
things are:
- Morten has reviewed your patch and had provided comments, mainly
pointing
towards the fact that the guarantees that the code is trying to
provide are
not met.
- You have replied/promised to review and update the patch (is that
correct?)
- Nothing has happened since in this area (is that true?)
both are true
To us, looking at the code (mainly the while loop in the
is_buddy_busy()
function), is seem that you are trying to guarantee that the
runnable_avg_sum
and the runnable_avg_period that you read from the runqueue have not
been
updated before deciding if that cpu is busy. But there is no reason*
why
another thread could not update the runnable_avg_sum and then get
preempted
before updating the runnable_avg_period, which means you will still
use the
wrong values when doing the "greater than" check in the return
statement.
No, I don't want to ensure that values have not been updated before I use them but that both values are coherent.
But that to me and others seems to be false. I've suggested a way that the values might not be coherent and yet your while loop will exit.
The other point is that even if your code is "broken" (i.e. you read
stale
values that still get you out of the while loop) the code works but your estimate of a CPU being busy is off by .... 10% ? In other words, having that while loop in is_buddy_busy() does not change the
behaviour of
the rest of the code. So the objection is on having the while loop and trying to claim guarantees that are not met. Why not removing the loop?
So if it doesn't change the behavior why removing it ? It make the maintenance of patches series more complex.
Because we don't like adding code that doesn't work as claimed and then justify its presence by arguing that even if it's broken it doesn't change behaviour.
Why having that code in the first place?
All we are suggesting here is that the while loop is broken in its assumptions and that you get the same behaviour if you remove it. If you agree then push a patch that removes the while loop and we are going to be likely in favor of the new patch being included. If you really want to have the values coherent then you need a better mechanism for ensuring that.
Even if that is the case, I'm afraid I don't quite like the way this was done. IMHO, you shouldn't just revert bits of another author's patches that you don't agree with.
If there are issues regarding the the patches from Vincent, I'd do the following things in order of priority: 1. Prove to him that the race exists, preferably with a reproducible test case 2. Give him a chance to convince you otherwise 3. Share test results that show bad things happen as a result of some code 4. Ask _him_ to separate out that bit from the original patch so you can only pick the bits you like
I haven't seen this happen. All I've seen is one side claim it can happen and the other claim that it can't. *shrug*
Viresh, as an experienced maintainer, I hope you see the value of this approach rather than just pull in the tree.
I realise we're all under pressure here. So let's take a deep breath, step back and do it the right way.
/Amit