Re: [Qemu-devel] [PATCH V8 07/11] NUMA: set guest numa nodes memory policy

[Andrew Jones <drjones@redhat.com>]

----- Original Message -----
> On 08/20/2013 09:41 PM, Andrew Jones wrote:
> >> +
> >> +    /* This is a workaround for a long standing bug in Linux'
> >> +     * mbind implementation, which cuts off the last specified
> >> +     * node. To stay compatible should this bug be fixed, we
> >> +     * specify one more node and zero this one out.
> >> +     */
> >> +    clear_bit(numa_num_configured_nodes() + 1, numa_info[i].host_mem);
> >> +    if (mbind(ram_ptr + ram_offset, len, bind_mode,
> >> +        numa_info[i].host_mem, numa_num_configured_nodes() + 1, 0)) {
> >> +            perror("mbind");
> >> +            return -1;
> >> +    }
> > 
> >>From my quick read of this patch series, I think these two calls of
> > numa_num_configured_nodes() are the only places that libnuma is used.
> > Is it really worth the new dependency? Actually libnuma will only calculate
> > what it returns from numa_num_configured_nodes() once, because it simply
> > counts bits in a bitmask that it only initializes at library load time. So
> > it would be more robust wrt to node onlining/offlining to avoid libnuma and
> > to just fetch information from sysfs as needed anyway. In this particular
> > code though, I think replacing numa_num_configured_nodes() with a maxnode,
> > where
> > 
> > unsigned long maxnode = find_last_bit(numa_info[i].host_mem,
> > MAX_CPUMASK_BITS)
> 
> Sorry I can't understand this since numa_numa_configured_nodes() is for host,
> but why could we find the last bit of guest setting to replace it?
> 

You're not using numa_numa_configured_nodes() to index _the_ host's nodemask,
you're using it to find the highest possible bit set in _a_ nodemask,
numa_info[i].host_mem. mbind doesn't need its 'maxnode' param to be
the highest possible host node bit, but rather just the highest possible bit
set in the nodemask passed to it. find_last_bit will find that bit. You still
need to add 1 to it as you do with numa_numa_configured_nodes() though, due
to the kernel decrementing it by one erroneously as you've pointed out in your
comment.

drew