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

[Wanlong Gao <gaowanlong@cn.fujitsu.com>]

On 08/21/2013 03:15 PM, Andrew Jones wrote:
> 
> 
> ----- 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.

Thank you very much for your explanation, I'll change as you said. ;)

Regards,
Wanlong Gao

> 
> drew
>