RE: [PATCH] hv_balloon: Add module parameter to configure balloon floor value

[Michael Kelley (LINUX)]

From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Tuesday, October 17, 2023 7:41 AM
> 
> Angelina Vu <angelinavu@xxxxxxxxxxxxxxxxxxx> writes:
> 
> > Currently, the balloon floor value is automatically computed, but may be
> > too small depending on app usage of memory. This patch adds a balloon_floor
> > value as a module parameter that can be used to manually configure the
> > balloon floor value.
> >
> > Signed-off-by: Angelina Vu <angelinavu@xxxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/hv/hv_balloon.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> > index 64ac5bdee3a6..87b312f99b2e 100644
> > --- a/drivers/hv/hv_balloon.c
> > +++ b/drivers/hv/hv_balloon.c
> > @@ -1101,6 +1101,10 @@ static void process_info(struct hv_dynmem_device *dm,
> struct dm_info_msg *msg)
> >  	}
> >  }
> >
> > +unsigned long balloon_floor;
> > +module_param(balloon_floor, ulong, 0644);
> > +MODULE_PARM_DESC(balloon_floor, "Memory level (in megabytes) that ballooning
> will not remove");
> > +
> >  static unsigned long compute_balloon_floor(void)
> >  {
> >  	unsigned long min_pages;
> > @@ -1117,6 +1121,9 @@ static unsigned long compute_balloon_floor(void)
> >  	 *    8192       744    (1/16)
> >  	 *   32768      1512	(1/32)
> >  	 */
> > +	if (balloon_floor)
> > +		return MB2PAGES(balloon_floor);
> > +
> >  	if (nr_pages < MB2PAGES(128))
> >  		min_pages = MB2PAGES(8) + (nr_pages >> 1);
> >  	else if (nr_pages < MB2PAGES(512))
> 
> A module parameter is probably useful for debugging but it can hardly be
> applied in production environments as it must be 'one size fits all' and
> e.g. for different VM sizes it can be different (that's the purpose of
> compute_balloon_floor() heuristics).
> 
> In fact, does it has to be statically set? Can we have a sysfs entity so
> this can be a policy (userspace decision)? We can keep the current
> compute_balloon_floor() as the default but users will be able to adjust
> accordingly.
> 

The module parameter can also be set or changed at runtime via
/sys/module/balloon/parameters/balloon_floor.  Changes made by
writing to that path are immediately reflected in the value of
the balloon_floor variable.  I think that accomplishes everything
you've outlined while also allowing a value to be set on the
kernel boot line.

Michael