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Re: [Xen-devel] Scheduler problem in XEN 3.4.0

Pankaj Parakh wrote:
Hi George,

Thanks for showing interest, First of all I'll try to setup debugging
environment for Xen, I also checked the mailing list of Ananth also I
know him personally, he has left it some where in middle.

I tried printk's and locking method to find out which function is
being called in schedule.c, I found the following sequence:-

(from here no function is called from schedule.c, but system hangs)

I cant say what the problem is, I am only changing the sched_priv data
of vcpu struct, also my rr sched has only three functions viz
init_vcpu, destroy_vcpu, do_schedule.

I have attached the code as well.

Following are my doubts:
[Language point: In many English dialects, such as US and UK, "doubt" implies something negative. To avoid being misunderstood by speakers of those dialects, "questions" might be a better word to use.]
1. Is the above function sequence is right ? Why two times init_domain
is called during the boot.
I suspect that the idle domain (32727) and domain 0 are the two domains being initialized. You could easily find out by adding the domain id to the printk.
2. Do a scheduler policy need to manipulate other part of vcpu struct
(other than sched_priv)
I don't believe so. You could look through other schedulers (like the credit scheduler) to see if this is so.
3. Is it necessary to maintain domain information inside scheduler policy
I don't know.

A quick scan through the attached file turn up a couple of points:
+ It appears that the algorithm adds *all* vcpus to a single global runqueue, and scans through them looking for "runnable" vcpus on schedule. This seems pretty pointless: why not add them to the global runqueue on wake? + Furthermore, I'm not sure your linked-list implementation is sound; for example, the initial v->sched_priv is not initialized to NULL. (Not going to spend a lot of time trying to figure out if that's OK or not.) + There is some missing logic regarding the v->processor field and sync_vcpu_execstate(). Xen is designed expecting a per-cpu runqueue with explicit migrations of vcpus between pcpus. One of the reasons for this is so that when switching between a vcpu and the idle domain, it doesn't actually need to do a full context switch. As a result, if you run a vcpu on one cpu, and then run it on another without an explicit sync, you may have stale data in the vcpu struct (and Xen will throw a BUG). + You don't implement a .wake() method. I think that Xen will wake() dom0 when it's ready, expecting the .wake() method to raise a SCHEDULE softirq on the appropriate pcpu (from which the .do_schedule() method is called). So no wake method means no schedule(), and no schedule means it just runs the idle domain.

It might be best to start with just one pcpu, and adding multiple cpus once you get things running. Try adding a wake() method that will check to see if cpu 0 is idle; if it is, raise SCHEDULE_SOFTIRQ on that cpu, and see what you get. After that, try adding some logic to figure out which other cpu to wake instead; but be advised that you'll probably hit the BUG() relating to sync_vcpu_execstate() not being properly called.

On Tue, Oct 13, 2009 at 5:17 PM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:

I haven't used the round-robin scheduler code in that book, but
another guy named Ananth tried to use it unsuccessfully as well.  You
can see some of that thread here (not sure why I can't find the
original post):

Most of us are not so interested in finding the bug in the books'
code, but we are interested in helping *you* find it.  If you continue
to do hypervisor work (and especially if you do anything with the
scheduler), you're going to have to learn how to debug a hypervisor,
which is often rather a pain in the neck.

There is some advice in the thread linked to above about setting up a
serial console.  You can add printk()'s around to find out what the
sched_rr code is doing and where it's going wrong, and ask more
questions on the list if you get stuck.  (Feel free to cc me to bring
it to my attention, but always send it to the whole list.)

Good luck,

On Mon, Oct 12, 2009 at 6:15 PM, Pankaj Parakh
<me.pankajparakh@xxxxxxxxx> wrote:
Hi All,

I am trying to learn about schedulers of XEN, so for a start I am
using XEN 3.4.0 and using book - The Definitive Guide to the Xen
Hypervisor - by David Chisnall, I have followed its steps to add
scheduler which is there in Chap. 12
But I dont know what is the problem, I am unable to boot with that
scheduler selected, I have been trying to debug this problem, but
kinda stuck in it.
The scheduler given in that book is a trivial round robin scheduler.

Is the problem is with the code or with the procedure, I dont know that also.

Plzz help me out of this.

Pankaj Parakh

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Pankaj Parakh

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