Re: [Xen-devel] [PATCH] xen/balloon: export balloon hotplug page stats to sysfs

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Mar 21, 2014 at 01:02:26PM +0000, David Vrabel wrote:
> On 20/03/14 14:01, Wei Liu wrote:
> > On Thu, Mar 20, 2014 at 01:21:17PM +0000, David Vrabel wrote:
> >> On 20/03/14 12:56, Wei Liu wrote:
> >>> --- a/drivers/xen/xen-balloon.c
> >>> +++ b/drivers/xen/xen-balloon.c
> >>> @@ -129,6 +129,11 @@ module_exit(balloon_exit);
> >>>  BALLOON_SHOW(current_kb, "%lu\n", PAGES2KB(balloon_stats.current_pages));
> >>>  BALLOON_SHOW(low_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_low));
> >>>  BALLOON_SHOW(high_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_high));
> >>> +#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
> >>> +BALLOON_SHOW(hotplug_kb, "%lu\n", PAGES2KB(balloon_stats.hotplug_pages));
> >>> +BALLOON_SHOW(balloon_hotplug_kb, "%lu\n",
> >>> +      PAGES2KB(balloon_stats.balloon_hotplug));
> >>> +#endif
> >>
> >> Extending the kernel's ABI requires justification.  How are these values
> >> useful for userspace tools?  If it's just for debug purposes, using
> >> debugfs would be preferred.
> >>
> > 
> > My original thought is that userspace tools have the right to know how
> > many pages balloon driver has. Missing the hotplug pages is not very
> > desirable.
> 
> What tools needs the value and how will it use it to make sensible
> decisions?
> 

I see this issue the other way around. Without this information tools
cannot make correct decision, because it cannot possibly know there even
exists hotplug pages in balloon driver. Adding this is not for the sake of
existing tool, but for the sake of correctness. If you think userspace
will never have the need to know this information then debugfs it is.

> If there isn't an answer and it just for diagnostics/debug then debugfs
> is the right place.
> 
> David

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