Re: [PATCH v2 8/8] drm/xe/gt_throttle: Avoid TOCTOU when monitoring reasons

[Rodrigo Vivi <rodrigo.vivi@intel.com>]

On Sun, Oct 26, 2025 at 10:57:20PM -0700, Lucas De Marchi wrote:
> It's currently not possible to safely monitor if there's throttling
> happening and what are the reasons. The approach of reading the status
> and then reading the reasons is not reliable as by the time sysadmin
> reads the reason, the throttling could not be happening anymore.
> 
> Previous tentative to fix that[1] was breaking the ABI and potentially
> sysadmin's scripts. This takes a different approach of adding and
> documenting the additional attribute. It's still valuable, though
> redundant, to provide the simpler 0/1 interface.
> 
> In order to avoid userspace knowledge on the bitmask meaning and to be
> able to maintain the kernel side in sync with possible changes in
> future, just walk the attribute group and check what are the masks that
> match the value read.
> 
> [1] https://lore.kernel.org/intel-xe/20241025092238.167042-1-raag.jadav@intel.com/
> 
> Cc: Raag Jadav <raag.jadav@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_gt_throttle.c | 46 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 44 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_gt_throttle.c b/drivers/gpu/drm/xe/xe_gt_throttle.c
> index fa7068aac3344..fd2988dacbbb6 100644
> --- a/drivers/gpu/drm/xe/xe_gt_throttle.c
> +++ b/drivers/gpu/drm/xe/xe_gt_throttle.c
> @@ -22,9 +22,15 @@
>   * Their availability depend on the platform and some may not be visible if that
>   * reason is not available.
>   *
> + * The ``status_reasons`` attribute can be used by sysadmin monitoring all
> + * possible reasons for throttling and reporting them. It's preferred over
> + * monitoring ``status`` and then reading the reason both for simplicity and to
> + * avoid TOCTOU.

Perhaps add something like: TOCTOU (time-of-check to time-of-use).

> + *
>   * The following attributes are available on Crescent Island platform:
>   *
> - * - ``status``: Overall throttle status
> + * - ``status``: Overall throttle status (0: no throttling, 1: throttling)
> + * - ``status_reasons``: All reasons causing throttling separated by newline.
>   * - ``reason_pl1``: package PL1
>   * - ``reason_pl2``: package PL2
>   * - ``reason_pl4``: package PL4
> @@ -43,7 +49,8 @@
>   *
>   * Other platforms support the following reasons:
>   *
> - * - ``status``: Overall status
> + * - ``status``: Overall throttle status (0: no throttling, 1: throttling)
> + * - ``status_reasons``: All reasons causing throttling separated by newline.
>   * - ``reason_pl1``: package PL1
>   * - ``reason_pl2``: package PL2
>   * - ``reason_pl4``: package PL4, Iccmax etc.
> @@ -111,12 +118,45 @@ static ssize_t reason_show(struct kobject *kobj,
>  	return sysfs_emit(buff, "%u\n", is_throttled_by(gt, ta->mask));
>  }
>  
> +static const struct attribute_group *get_platform_throttle_group(struct xe_device *xe);
> +
> +static ssize_t status_reasons_show(struct kobject *kobj,
> +				   struct kobj_attribute *attr, char *buff)
> +{
> +	struct xe_gt *gt = throttle_to_gt(kobj);
> +	struct xe_device *xe = gt_to_xe(gt);
> +	const struct attribute_group *group;
> +	struct attribute **pother;
> +	ssize_t ret = 0;
> +	u32 reasons;
> +
> +	reasons = xe_gt_throttle_get_limit_reasons(gt);
> +	group = get_platform_throttle_group(xe);
> +
> +	for (pother = group->attrs; *pother; pother++) {
> +		struct kobj_attribute *kattr = container_of(*pother, struct kobj_attribute, attr);
> +		struct throttle_attribute *other_ta = kobj_attribute_to_throttle(kattr);
> +
> +		if (other_ta->mask != U32_MAX && reasons & other_ta->mask)
> +			ret += sysfs_emit_at(buff, ret, "%s\n", (*pother)->name);

perhaps a space instead of the \n to keep only a single line?

> +	}
> +
> +	return ret;
> +}
> +
>  #define THROTTLE_ATTR_RO(name, _mask)				\
>  	struct throttle_attribute attr_##name =	{		\
>  		.attr = __ATTR(name, 0444, reason_show, NULL),	\
>  		.mask = _mask,					\
>  	}
>  
> +#define THROTTLE_ATTR_RO_FUNC(name, _mask, _show)		\
> +	struct throttle_attribute attr_##name =	{		\
> +		.attr = __ATTR(name, 0444, _show, NULL),	\
> +		.mask = _mask,					\
> +	}
> +
> +static THROTTLE_ATTR_RO_FUNC(status_reasons, 0, status_reasons_show);
>  static THROTTLE_ATTR_RO(status, U32_MAX);
>  static THROTTLE_ATTR_RO(reason_pl1, POWER_LIMIT_1_MASK);
>  static THROTTLE_ATTR_RO(reason_pl2, POWER_LIMIT_2_MASK);
> @@ -128,6 +168,7 @@ static THROTTLE_ATTR_RO(reason_vr_thermalert, VR_THERMALERT_MASK);
>  static THROTTLE_ATTR_RO(reason_vr_tdc, VR_TDC_MASK);
>  
>  static struct attribute *throttle_attrs[] = {
> +	&attr_status_reasons.attr.attr,
>  	&attr_status.attr.attr,
>  	&attr_reason_pl1.attr.attr,
>  	&attr_reason_pl2.attr.attr,
> @@ -153,6 +194,7 @@ static THROTTLE_ATTR_RO(reason_psys_crit, PSYS_CRIT_MASK);
>  
>  static struct attribute *cri_throttle_attrs[] = {
>  	/* Common */
> +	&attr_status_reasons.attr.attr,
>  	&attr_status.attr.attr,
>  	&attr_reason_pl1.attr.attr,
>  	&attr_reason_pl2.attr.attr,
> 
> -- 
> 2.51.0
>