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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Mon, Jul 27, 2026 at 11:05:45PM +0000, pratmal@google.com wrote:
> From: Pratyush Mallick <pratmal@google.com>
>
> 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?

>
> 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.

Have you considered actually adding logic to detect the problem you're having
and delay in that case?

It seems like this is the "easy" solution but it has a price too.

>
> Signed-off-by: Pratyush Mallick <pratmal@google.com>
> ---
> v3:
>  - Converted page_reporting_delay_ms from a sysctl to a module parameter.
>  - Dropped the max value cap (PAGE_REPORTING_DELAY_MS_MAX).
>  - Documented page_reporting_delay_ms in kernel-parameters.txt.
>  - Updated code comments in mm/page_reporting.c.
>  - v2: https://lore.kernel.org/linux-mm/3da27fde-25dc-4cb8-8e05-74cd26fc2f7c@kernel.org/T/#t
>
> v2:
>  - Documented page_reporting_delay_ms in Documentation/admin-guide/sysctl/vm.rst.
>  - v1: https://lore.kernel.org/linux-mm/20260722192935.1646848-1-pratmal@google.com/T/#u
>
> v1: Fixed feedback from RFC.
>  - Added lower and upper cap to sysctl value.
>  - Reverted the reordering on page_reporting_delay_ms.
>  - Dropped the mod_delayed_work() change.
>  - RFC: https://lore.kernel.org/linux-mm/20260714171456.2350037-1-pratmal@google.com/T/#u
>  .../admin-guide/kernel-parameters.txt         |  6 ++++++
>  mm/page_reporting.c                           | 21 ++++++++++++-------
>  2 files changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b5493a7f8f22..364c2dce8e70 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4810,6 +4810,12 @@ Kernel parameters
>  			Adjust the minimal page reporting order. The page
>  			reporting is disabled when it exceeds MAX_PAGE_ORDER.
>
> +	page_reporting.page_reporting_delay_ms=
> +			[KNL] Free page reporting delay in milliseconds
> +			Format: <unsigned integer>
> +			Adjust the delay in milliseconds between free page
> +			reporting intervals. Default is 2000 (2 seconds).
> +
>  	panic=		[KNL] Kernel behaviour on panic: delay <timeout>
>  			timeout > 0: seconds before rebooting
>  			timeout = 0: wait forever
> diff --git a/mm/page_reporting.c b/mm/page_reporting.c
> index 942e84b6908a..a67311468204 100644
> --- a/mm/page_reporting.c
> +++ b/mm/page_reporting.c
> @@ -47,7 +47,11 @@ MODULE_PARM_DESC(page_reporting_order, "Set page reporting order");
>   */
>  EXPORT_SYMBOL_GPL(page_reporting_order);
>
> -#define PAGE_REPORTING_DELAY	(2 * HZ)
> +static unsigned int page_reporting_delay_ms = 2 * MSEC_PER_SEC;
> +module_param(page_reporting_delay_ms, uint, 0644);
> +MODULE_PARM_DESC(page_reporting_delay_ms,
> +		 "Set page reporting delay in milliseconds");
> +
>  static struct page_reporting_dev_info __rcu *pr_dev_info __read_mostly;
>
>  enum {
> @@ -76,11 +80,11 @@ __page_reporting_request(struct page_reporting_dev_info *prdev)
>  		return;
>
>  	/*
> -	 * 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?

>  }
>
>  /* notify prdev of free page reporting request */
> @@ -335,12 +339,13 @@ static void page_reporting_process(struct work_struct *work)
>  err_out:
>  	/*
>  	 * If the state has reverted back to requested then there may be
> -	 * additional pages to be processed. We will defer for 2s to allow
> -	 * more pages to accumulate.
> +	 * additional pages to be processed. We will defer by
> +	 * page_reporting_delay_ms to allow more pages to accumulate.
>  	 */
>  	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?

>  }
>
>  static DEFINE_MUTEX(page_reporting_mutex);
> --
> 2.55.0.229.g6434b31f56-goog
>

Thanks, Lorenzo