Xen devel - Jun 2012 - [PATCH] xl: improve the comments for sedf parameters checking

As agreed during review of what has become 513d5e196e23.

No real functional changes.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>

diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
--- a/tools/libxl/xl_cmdimpl.c
+++ b/tools/libxl/xl_cmdimpl.c
@@ -565,28 +565,40 @@ static int sched_params_valid(libxl_doma
     if (sci.sched == LIBXL_SCHEDULER_SEDF) {
         if (has_weight && (has_period || has_slice))
             return 0;
+        /* If you want a real-time domain, with its own period and
+         * slice, please, do provide both! */
         if (has_period != has_slice)
             return 0;
 
         /*
          * Idea is, if we specify a weight, then both period and
-         * slice has to be zero. OTOH, if we do not specify a weight,
-         * that means we want a pure best effort domain or an actual
-         * real-time one. In the former case, it is period that needs
-         * to be zero, in the latter, weight should be.
+         * slice has to be zero. OTOH, if we do specify a period and
+         * slice, it is weight that should be zeroed. See
+         * docs/misc/sedf_scheduler_mini-HOWTO.txt for more details
+         * on the meaningful combinations and their meanings.
          */
         if (has_weight) {
             scp->slice = 0;
             scp->period = 0;
         }
         else if (!has_period) {
-            /* We can setup a proper best effort domain (extra time only)
-             * iff we either already have or are asking for some extra time. */
+            /* No weight nor slice/period means best effort. Parameters needs
+             * some mangling in order to properly ask for that, though. */
+
+            /*
+             * Providing no weight does not make any sense if we do not allow
+             * the domain to run in extra time. On the other hand, if we have
+             * extra time, weight will be ignored (and zeroed) by Xen, but it
+             * can''t be zero here, or the call for setting the
scheduling
+             * parameters will fail. So, avoid the latter by setting a random
+             * weight (namely, 1), as it will be ignored anyway.
+             */
             scp->weight = has_extratime ? scp->extratime : sci.extratime;
             scp->period = 0;
-        }
-        if (has_period && has_slice)
+        } else {
+            /* Real-time domain: will get slice CPU time over every period */
             scp->weight = 0;
+       }
     }
 
     return 1;

Dario Faggioli writes ("[Xen-devel] [PATCH] xl: improve the comments for
sedf parameters checking"):
> As agreed during review of what has become 513d5e196e23.
Thanks.  I''m afraid it doesn''t apply to tip though.

Ian.

On Fri, 2012-06-29 at 17:29 +0100, Ian Jackson wrote:
> Dario Faggioli writes ("[Xen-devel] [PATCH] xl: improve the comments
for sedf parameters checking"):
> > As agreed during review of what has become 513d5e196e23.
> 
> Thanks.  I''m afraid it doesn''t apply to tip though.
This code got moved to libxl while we were trying to fixup this stuff
and the equivalent to this patch was applied to libxl as part of
25494:836db8c4b9f9 "libxl: fix validation of scheduling parameters for
sedf".

Ian.

Ian Campbell writes ("Re: [Xen-devel] [PATCH] xl: improve the comments for
sedf parameters	checking"):
> On Fri, 2012-06-29 at 17:29 +0100, Ian Jackson wrote:
> > Dario Faggioli writes ("[Xen-devel] [PATCH] xl: improve the
comments for sedf parameters checking"):
> > > As agreed during review of what has become 513d5e196e23.
> > 
> > Thanks.  I''m afraid it doesn''t apply to tip though.
> 
> This code got moved to libxl while we were trying to fixup this stuff
> and the equivalent to this patch was applied to libxl as part of
> 25494:836db8c4b9f9 "libxl: fix validation of scheduling parameters for
> sedf".
Oh good.

Ian.

On Mon, 2012-07-02 at 11:36 +0100, Ian Jackson wrote: 
> Ian Campbell writes ("Re: [Xen-devel] [PATCH] xl: improve the comments
for sedf parameters	checking"):
> > On Fri, 2012-06-29 at 17:29 +0100, Ian Jackson wrote:
> > > Dario Faggioli writes ("[Xen-devel] [PATCH] xl: improve the
comments for sedf parameters checking"):
> > > > As agreed during review of what has become 513d5e196e23.
> > > 
> > > Thanks.  I''m afraid it doesn''t apply to tip
though.
> > 
> > This code got moved to libxl while we were trying to fixup this stuff
> > and the equivalent to this patch was applied to libxl as part of
> > 25494:836db8c4b9f9 "libxl: fix validation of scheduling
parameters for
> > sedf".
> 
Indeed. :-)
> Oh good.
> 
Yep, sorry for not having replied soon, I was on leave from Firday
afternoon to yesterday (included).

Thanks and Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)




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