On Fri, Sep 18, 2026 at 01:43:21PM +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.
>
> v2:
> - Changed workqueue from deprecated system_unbound_wq to system_dfl_wq.
> (Arvind Yadav)
>
> Fixes: a26084328ac4 ("drm/pagemap, drm/xe: Manage drm_pagemap provider
> lifetimes")
> Reviewed-by: Arvind Yadav <[email protected]>
> Reviewed-by: Thomas Hellström <[email protected]>
> Signed-off-by: Varun Gupta <[email protected]>
Did this patch break our CI [1] [2]? Varun can you take a look at a
follow up or work with CI so this isn't flagged?
Thanks,
Matt
[1]
https://intel-gfx-ci.01.org/tree/intel-xe/xe-pw-174512v1/bat-bmg-2/igt@[email protected]
[2]
https://intel-gfx-ci.01.org/tree/intel-xe/xe-pw-174512v1/bat-bmg-vm/igt@[email protected]
> ---
> 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..0867bfbde5c5 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_dfl_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();
> }
> }
>
> --
> 2.43.0
>
