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

["Yadav, Arvind" <arvind.yadav@intel.com>]

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On 18-09-2026 12:35, Thomas Hellström wrote:
> 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);


Instead of system_unbound_wq we should use system_dfl_wq because 
system_unbound_wq is deprecated.
Apart from this LGTM:

Reviewed-by: Arvind Yadav <arvind.yadav@intel.com>


>>   	/*
>>   	 * 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();
>>   	}
>>   }
>>   

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