Re: [PATCH v10 3/6] mm: Introduce Reported pages

Michael S. Tsirkin
2019-09-23T15:49:13Z

On Mon, Sep 23, 2019 at 05:45:29PM +0200, David Hildenbrand wrote:
> On 23.09.19 17:37, Michael S. Tsirkin wrote:
> > On Mon, Sep 23, 2019 at 08:28:00AM -0700, Alexander Duyck wrote:
> >> On Mon, Sep 23, 2019 at 8:00 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >>>
> >>> On Mon, Sep 23, 2019 at 07:50:15AM -0700, Alexander Duyck wrote:
> >>>>>> +static inline void
> >>>>>> +page_reporting_reset_boundary(struct zone *zone, unsigned int order, int mt)
> >>>>>> +{
> >>>>>> + int index;
> >>>>>> +
> >>>>>> + if (order < PAGE_REPORTING_MIN_ORDER)
> >>>>>> + return;
> >>>>>> + if (!test_bit(ZONE_PAGE_REPORTING_ACTIVE, &zone->flags))
> >>>>>> + return;
> >>>>>> +
> >>>>>> + index = get_reporting_index(order, mt);
> >>>>>> + reported_boundary[index] = &zone->free_area[order].free_list[mt];
> >>>>>> +}
> >>>>>
> >>>>> So this seems to be costly.
> >>>>> I'm guessing it's the access to flags:
> >>>>>
> >>>>>
> >>>>> /* zone flags, see below */
> >>>>> unsigned long flags;
> >>>>>
> >>>>> /* Primarily protects free_area */
> >>>>> spinlock_t lock;
> >>>>>
> >>>>>
> >>>>>
> >>>>> which is in the same cache line as the lock.
> >>>>
> >>>> I'm not sure what you mean by this being costly?
> >>>
> >>> I've just been wondering why does will it scale report a 1.5% regression
> >>> with this patch.
> >>
> >> Are you talking about data you have collected from a test you have
> >> run, or the data I have run?
> >
> > About the kernel test robot auto report that was sent recently.
>
> https://lkml.org/lkml/2019/9/21/112
>
> And if I'm correct, that regression is observable in case reporting is
> not enabled. (so with this patch applied only, e.g., on a bare-metal system)

Exactly. That's what makes it interesting.

>
> --
>
> Thanks,
>
> David / dhildenb