"Michael Kelley (LINUX)" <mikel...@microsoft.com> writes:
> From: Vitaly Kuznetsov <vkuzn...@redhat.com> Sent: Tuesday, October 17, 2023
> 7:41 AM
>>
>> Angelina Vu <angelin...@linux.microsoft.com> 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 <angelin...@linux.microsoft.com>
>> > ---
>> > 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.
Oh, thanks, I've actually forgot it is r/w. What's IMO not ideal with
this approach is that if you don't pass any value on the kernel command
line, '/sys/module/balloon/parameters/balloon_floor' will contain '0' so
the user will have to guess the actual current value. Would it make
sense to do:
if (!balloon_floor)
balloon_floor = compute_balloon_floor();
on boot/whenever(if) totalram_pages() changes? Personally, I'm not sure
it's a good idea as I've never seen kernel module parameters which would
behave this way :-) Another thing is that I'm not sure to which extent
'/sys/module/*/parameters/' can be considered a stable ABI, i.e. it is
not documented like Documentation/ABI/stable/*.
--
Vitaly
