Re: Test result of xen-unstable changeset 25249

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Fri, 2012-04-27 at 10:08 +0100, Ian Campbell wrote: 
> > I think what we really want to do is is any of the parameters are set, 
> > after the domain is first created, to read the scheduling parameters for 
> > the domain (which will be the defaults), change the ones that are set in 
> > the config file, and then write the whole structure back.  That 
> > shouldn't be too hard, as libxl__sched_set_params() is called after the 
> > domain itself is created;
> 
> I guess the low level libxl_sched_*_params_set should take care of this?
> 
Possibly, but there's more I'm not sure I understand in the patch (the
original patch, the one that has been checked-in on Wednesday):

+int libxl__sched_set_params(libxl__gc *gc, uint32_t domid, libxl_sched_params *scparams)
+{
+    libxl_ctx *ctx = libxl__gc_owner(gc);
+    libxl_scheduler sched;
+    libxl_sched_sedf_domain sedf_info;
+    libxl_sched_credit_domain credit_info;
+    libxl_sched_credit2_domain credit2_info;
+    int ret;
+
+    sched = libxl_get_scheduler (ctx);
+    switch (sched) {
+    case LIBXL_SCHEDULER_SEDF:
+      sedf_info.period = scparams->period;
+      sedf_info.slice = scparams->slice;
+      sedf_info.latency = scparams->latency;
+      sedf_info.extratime = scparams->extratime;
+      sedf_info.weight = scparams->weight;
+      ret=libxl_sched_sedf_domain_set(ctx, domid, &sedf_info);
+      break;
+    case LIBXL_SCHEDULER_CREDIT:
+      credit_info.weight = scparams->weight;
<snip>

sched gets the return value of libxl_get_scheduler() which, if I'm not
wrong , read the "default" xen scheduler for this boot, i.e., the one
specified by the "sched=" boot command line or whatever the default for
that is (credit1).

After that it decides what libxl_sched_*_domain_set() to call basing
right on that value. What I'm missing is what prevents the domain in
question to be part of a cpupool (e.g., by specifying "poolid=" in its
config file as well) for which the scheduler is a different one.

It seems to me that, in such case, we will be setting the wrong set of
parameters anyway, independently on how well we manage in putting a
default in place for them... Am I missing something? If not, as I
haven't found any way of finding out what scheduler is actually being
used for a specific domain, shouldn't we add or mimic that (going
through cpupool, perhaps, I haven't checked yet)?

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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