Re: [PATCH] drm/pagemap: Prevent CPU stalls during unbounded pagemap teardown

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2026-09-17 at 22:33 +0530, Varun Gupta wrote:
> drm_pagemap_dev_unhold_work() processes an unbatched llist of
> pagemaps
> on the system workqueue. This creates potential latency traps during
> heavy teardown cycles due to two compounding factors:
> 
> 1. The loop contains a per-item drm_dbg() trace. If dynamic debug is
> enabled on a slow serial console, synchronous print latency scales
> linearly with the batch size and can block worker execution.
> 2. The llist can accumulate large batches during intensive unmap
> operations, monopolizing worker resources without yielding.
> 
> This causes below log warning:
> "workqueue: drm_pagemap_dev_unhold_work [drm_gpusvm_helper] hogged
> CPU
> for >10000us 19 times, consider switching to WQ_UNBOUND"
> 
> Fix this by implementing a latency mitigation strategy:
> - Move the teardown work to system_unbound_wq to avoid tying up
>   per-CPU bound worker pools.
> - Add cond_resched() inside the teardown loop to yield the CPU during
>   large batch teardowns, ensuring system responsiveness.
> - Convert the drm_dbg() trace to drm_dbg_ratelimited() to mitigate
>   serial console bottlenecks while preserving debuggability.
> 
> Fixes: a26084328ac4 ("drm/pagemap, drm/xe: Manage drm_pagemap
> provider lifetimes")
> Signed-off-by: Varun Gupta <varun.gupta@intel.com>

LGTM.
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

> ---
>  drivers/gpu/drm/drm_pagemap.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_pagemap.c
> b/drivers/gpu/drm/drm_pagemap.c
> index 892b325fa99b..f8d5428f1750 100644
> --- a/drivers/gpu/drm/drm_pagemap.c
> +++ b/drivers/gpu/drm/drm_pagemap.c
> @@ -986,7 +986,7 @@ static void drm_pagemap_release(struct kref *ref)
>  	dpagemap->dev_hold = NULL;
>  	drm_pagemap_shrinker_add(dpagemap);
>  	llist_add(&dev_hold->link, &drm_pagemap_unhold_list);
> -	schedule_work(&drm_pagemap_work);
> +	queue_work(system_unbound_wq, &drm_pagemap_work);
>  	/*
>  	 * Here, either the provider device is still alive, since if
> called from
>  	 * page_free(), the caller is holding a reference on the
> dev_pagemap,
> @@ -1009,10 +1009,11 @@ static void
> drm_pagemap_dev_unhold_work(struct work_struct *work)
>  		struct drm_device *drm = dev_hold->drm;
>  		struct module *module = drm->driver->fops->owner;
>  
> -		drm_dbg(drm, "Releasing reference on provider device
> and module.\n");
> +		drm_dbg_ratelimited(drm, "Releasing reference on
> provider device and module.\n");
>  		drm_dev_put(drm);
>  		module_put(module);
>  		kfree(dev_hold);
> +		cond_resched();
>  	}
>  }
>