Re: [Xen-devel] [PATCH for 4.7 0/4] Assorted scheduling fixes

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 03/05/16 22:46, Dario Faggioli wrote:
> Hi,
> 
> This small series contains some bugfixes for various schedulers. They're all
> bugfixes, so I think all should be considered for 4.7. Here's some more
> detailed analysis.
> 
> Patch 1 and 3 are for Credit2. Patch 1 is a lot more important, as we have an
> ASSERT triggering without it. Patch 2 is behavioral fixing, which I believe it
> is important, but at least does not make anything explode.
> 
> Patch 2 fixes another ASSERT, in case a pCPU fails to come up. This is what
> Julien reported here:
> 
>  https://www.mail-archive.com/xen-devel@xxxxxxxxxxxxx/msg65918.html
> 
> Julien, the patch is very very similar to the one attached to one of my reply
> in that thread, but I had to change some small bits... Can you please re-test
> it?
> 
> Patch 4 makes the code of RTDS look consistent with what we state in patch 2,
> so it's also important. Furthermore, it does fix a bug (although, again, not
> one that would splat Xen) as, without it, we may have a timer used by the RTDS
> scheduler bound to the pCPU of another cpupool with another scheduler. That
> would introduce some unwanted and very difficult to recognize interference
> between different schedulers in different pool, and should hence be avoided.
> 
> So this was awesomeness; about risks:
>  - patch 1 is very small, super-self contained (zero impact outside of Credit2
>    code) and it fixes an actual and 100% reproducible bug;
>  - patch 2 is also totally self-contained and it can't possibly cause problems
>    to anything else than to what it is trying to fix (Credit2's load 
> balancer).
>    It doesn't cure any ASSERT or Oops, so it's less interesting, but given the
>    low risk --also considering that Credit2 will still be considered
>    experimental in 4.7-- I think it can go in;
>  - patch 3 is bigger, and a bit more complex. Note, however, that most of its
>    content is code comments and ASSERT-s; it is self contained to scheduling
>    (in the sense that it impacts all schedulers, but "just" them), and fixes
>    a situation that, AFAIUI, is important for ARM;

I think you reordered patch 2 and 3 between the time you wrote this and
the time you posted it. :-)

>  - patch 4 may again look not that critical. But, the fact that someone 
> wanting
>    to experiment with RTDS in a cpupool would face the kind of interference
>    between independent cpupools that the patch cures is, I think, something
>    worthwhile trying to avoid. Besides, it is again quite self contained, as
>    it's indeed only relevant for RTDS (which is also going to be called
>    experimental for 4.7).

FWIW I agree with the rationale here.

 -George


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