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Re: [Xen-devel] [PATCH 1/1] Improved RTDS scheduler

On Mon, 2016-01-04 at 21:48 +0800, Meng Xu wrote:
> On Mon, Jan 4, 2016 at 7:42 PM, Ian Campbell <ian.campbell@xxxxxxxxxx
> > wrote:
> > On Thu, 2015-12-31 at 04:45 -0500, Tianyang Chen wrote:
> > > Budget replenishment is now handled by a dedicated timer which is
> > > triggered at the most imminent release time of all runnable
> > > vcpus.
> > > 
> > > Signed-off-by: Tianyang Chen <tiche@xxxxxxxxxxxxxx>
> > > Signed-off-by: Meng Xu <mengxu@xxxxxxxxxxxxx>
> > > Signed-off-by: Dagaen Golomb <dgolomb@xxxxxxxxxxxxxx>
> > > ---
> > > Â0000-cover-letter.patchÂÂÂÂÂÂÂÂÂÂÂÂ|ÂÂÂ16 +++
> > > Â0001-Improved-RTDS-scheduler.patch |ÂÂ280
> > > ++++++++++++++++++++++++++++++++++++
> > 
> > I assume you didn't actually intend to commit those ;-)
> No. This version was sent by mistake. Please ignore this version. :-(
> Tianyang sent another version (v2) with the subject "[PATCH V2 1/1]
> Improved RTDS scheduler".
Indeed. BTW, I'm back today from Winter Holidays. I'll give a look at
this series as soon as practical.

I'll comment directly on V2, with the only exception of this very

So, first of all, Cc list (which is the same here and in V2): how have
the names of the people that are in there been picked?

The patch is only about xen/common/sched_rt.c. As per MAINTAINERS, the
maintainer of the code hosted in that file is just me. Surely it's a
good thing to also include George, as he's the other maintainer of
scheduling in general.

All the other people, though, should not be bothered by being copied
directly... Especially IanC and IanJ, I'd say.

> > (In general single patches do not require a separate cover letter
> > unless
> > the required context contains a large amount of information which
> > is not
> > appropriate for the commit message of the actual change, I'll leave
> > it to
> > you and the scheduler maintainers to decide how much of your cover
> > letter
> > it would be appropriate to move to the commit message).
> The cover letter is supposed to explain the design idea of the
> improved RTDS scheduler so that reviewers could (potentially) save
> time in reviewing the patch, we think. :-)
> Probably, the commit message should be refined and self-contained?
That's true. In particular, the "context", defined as summary of what
happened and have been discussed on the mailing list before, during
previous submissions, etc., definitely belongs to a cover letter.

High level design ideas and concepts, also do, e.g., in order to avoid
ending up with commit changelogs that are all several pages long! :-)

All that being said, we certainly want patches' subject lines and
changelogs to be descriptive of at least what is being done, and why.
If we commit this as it is right now, here it is what we'll find in
`git log':

 Improved RTDS scheduler

 Budget replenishment is now handled by a dedicated timer which is
 triggered at the most imminent release time of all runnable vcpus.

Which won't, I'm quite sure about it, be that much useful when looking
at it, say, in 3 years! :-D

I know, finding the proper balance of what's better put where is not
always super easy, especially at the beginning... but that's part of
the process of cooking a good patch (series) :-)

A few suggestions:

Â- we want tags in the subject. E.g., in this case, something like
 Â"xen: sched: Improved RTDS scheduler"

Â- avoid (in both subject and changelog) generic terms like "improve"
 Â"fix", etc., unless you can explain more specifically to what theyÂ
 Ârefer... I mean, pretty much all commits that touch sched_rt.c canÂ
 Âhave "Improve RTDS" as a subject, can't they? (Unless we decide toÂ
 Âcommit something making things deliberately worse!! :-))

 ÂIn this case, e.g. (although, perhaps a bit long):
  Â"xen: sched: convert RTDS from time to event driven model"

Â- as Ian said, there are information in the cover letter that can
 Âwell be moved in the changelog, making it a lot more useful, and
 Ânot less refined or self contained.

Thanks and Regards,
<<This happens because I choose it to happen!>> (Raistlin Majere)
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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