Re: [Xen-devel] [PATCH v20 10/10] tools: CMDs and APIs for Cache Monitoring Technology

[Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>]

On Fri, Oct 03, 2014 at 01:49:56PM +0100, Wei Liu wrote:
> Thanks for this quick turnaround.
> 
> Overall this looks good to me. Just some more questions on one thing I
> don't understand.
> 
> On Fri, Oct 03, 2014 at 07:55:15PM +0800, Chao Peng wrote:
> [...]
> > +int libxl__pick_random_socket_cpu(libxl__gc *gc, uint32_t socketid)
> > +{
> 
> This name is clearer.
> 
> But still, why randomization is required?
> 
> Does this mean picking arbitrary CPU returns the same result to library
> user? If so, why randomization is required?

The background here is that the L3 cache info we want to get in this
patch serial is a per-socket resource. To get it, we need to run the
related RDMSR from a cpu in that socket. So our real purpose of this
routine is to pick up a cpu number in that socket. From function
perspective, any cpu in that socket should work.

But for different domains we may have more than one
getting-l3-cache-info operations for a certain socket. We want to avoid
to run all these operations always on a same cpu every time. So the
randomization is used for load-balance among all the cpus in the same
socket.


> 
> > +    int i, j, cpu, nr_cpus;
> > +    libxl_cputopology *topology;
> > +    int *socket_cpus;
> > +
> > +    topology = libxl_get_cpu_topology(CTX, &nr_cpus);
> > +    if (!topology)
> > +        return ERROR_FAIL;
> > +
> > +    socket_cpus = libxl__malloc(gc, sizeof(int) * nr_cpus);
> > +
> > +    for (i = 0, j = 0; i < nr_cpus; i++)
> > +        if (topology[i].socket == socketid)
> > +            socket_cpus[j++] = i;
> > +
> > +    /* load balance among cpus in the same socket. */
> > +    cpu = socket_cpus[rand() % j];
> > +
> 
> There is one bug I can see. If socketid is not a valid id, j is not
> incremented. Then here you will have "divided by 0" error.
> 
> Although this is internal function, I don't see any sanity check in the
> public function that calls into this helper. In some instances it's even
> the first helper function that gets called.
> 
> My suggestion is that you make sure j is not 0 before dividing; return
> ERROR_INVAL otherwise.

Good catch, thanks Wei.
Will fix in next version.

Chao


> 
> 
> > +    libxl_cputopology_list_free(topology, nr_cpus);
> 
> (of course, don't forget to call this in error path as well)
> 
> Wei.
> 
> > +    return cpu;
> > +}
> > +
> 
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