Re: [PATCH v3] mm/page_reporting: Add page_reporting_delay_ms module parameter

[Pratyush Mallick]

On Tue, Jul 28, 2026 at 11:55 AM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 7/28/26 13:13, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Jul 27, 2026 at 11:05:45PM +0000, pratmal@xxxxxxxxxx wrote:
> >> From: Pratyush Mallick <pratmal@xxxxxxxxxx>
> >>
> >> Currently, the free page reporting uses a hardcoded delay of
> >> (2 HZ) between reporting intervals. While this is a reasonable
> >> default, it lacks the flexibility to adapt to varying guest workloads.
> >>
> >> A low delay allows aggressive memory reclamation, returning unused
> >> pages to the host as quickly as possible. However, during spiky
> >> allocation/free churn, this immediate reporting can lead to a severe
> >> performance penalty (nested page faults) as the guest re-allocates memory
> >> that the host has just unmapped. In these scenarios, there is benefit
> >> from increasing the delay to batch free pages over a longer window,
> >> absorbing the churn without hypercall and re-fault overhead.
> >
> > Since you're talking about increasing it, maybe set the floor at the current
> > value of 2s?

We benefit equally by setting the value to zero as well. This would immediately
report the pages being freed back to the host without any delay. This helps when
the host is under pressure, as any immediate memory release would help it.

> >>
> >> This patch exposes the delay as a module parameter:
> >> /sys/module/page_reporting/parameters/page_reporting_delay_ms, measured
> >> in milliseconds and defaults to 2000ms.
> >
> > I'm not sure this is great as it means we now have a parameter we have to
> > support forever and autotuning becomes harder to implement, also if later the
> > implementation is changed, this might prevent a reimplementation.
>
> While I'd prefer to keep the implementation as simple as possible unless really
> required, I guess auto-tune could be enabled by setting the parameter to e.g.,
> -1 in the future.

The case we're trying to solve involves tuning free page reporting based on host
memory pressure. I think the host is the primary beneficiary of free page
reporting, so if we wanted to dynamically autotune this delay, we would need
some mechanism from the host to enlighten the guest about its current memory
pressure and the autotuner would build its heuristics around it.
As far as I understand, no such mechanism currently exists. While
we do have ballooning, it requires the guest to actively allocate memory under
pressure, which makes it a comparatively slower mechanism and wouldn't help
much for simply adjusting the reporting rate of already free pages.

> >>      /*
> >> -     * Delay the start of work to allow a sizable queue to build. For
> >> -     * now we are limiting this to running no more than once every
> >> -     * couple of seconds.
> >> +     * Delay the start of work to allow a sizable queue to build.
> >> +     * We limit this based on page_reporting_delay_ms.
> >>       */
> >> -    schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
> >> +    schedule_delayed_work(&prdev->work,
> >> +                          msecs_to_jiffies(page_reporting_delay_ms));
> >
> > Err, do we not want to limit this to something sensible? What if the user
> > specifies 0 does it just hammer the system at that stage?
>
> IIUC, 0 just means "as soon there is a suitable free page block to report, start
> reporting immediately".
>
> With 2s, we wait 2s before we start reporting immediately by kicking the
> workqueue immediately.
>
> So "0" does not mean "report all the time", rather "start reporting immediately
> as we are notified about a pageblock to report".

Right. Setting the delay to 0 does not hammer the system. The execution
is constrained by two checks:
1. The worker is only scheduled when high-order pages (default is >= 2MB) are
freed. This makes scheduling relatively infrequent compared to normal 4K
page allocations.
2. Even when the worker wakes up, the actual expensive operations, such
as page isolation, acquiring locks, and hypercalls to report pages, are
guarded by the watermark check in page_reporting_process_zone(). We only
proceed with reporting if the zone has enough free pages to fill the
capacity batches (32 slots by default). If the watermark is not met,
the worker simply
exits and returns to the IDLE state without issuing any hypercalls.

> >>       state = atomic_cmpxchg(&prdev->state, state, PAGE_REPORTING_IDLE);
> >>       if (state == PAGE_REPORTING_REQUESTED)
> >> -             schedule_delayed_work(&prdev->work, PAGE_REPORTING_DELAY);
> >> +             schedule_delayed_work(&prdev->work,
> >> +                                   msecs_to_jiffies(page_reporting_delay_ms));
> >
> > This code is duplicated, while you've making this change maybe pull this into
> > its own function?
> >

Thanks. I can create a new function for this and send a V4.

Regards,
Pratyush