Re: [PATCH] mm/page_alloc: Occasionally relinquish zone lock in batch freeing

[Andrew Morton]

On Mon, 18 Aug 2025 11:58:03 -0700 Joshua Hahn <joshua.hahnjy@xxxxxxxxx> wrote:

> While testing workloads with high sustained memory pressure on large machines
> (1TB memory, 316 CPUs), we saw an unexpectedly high number of softlockups.
> Further investigation showed that the lock in free_pcppages_bulk was being held
> for a long time, even being held while 2k+ pages were being freed.
> 
> Instead of holding the lock for the entirety of the freeing, check to see if
> the zone lock is contended every pcp->batch pages. If there is contention,
> relinquish the lock so that other processors have a change to grab the lock
> and perform critical work.
> 
> In our fleet,

who is "our"?

> we have seen that performing batched lock freeing has led to
> significantly lower rates of softlockups, while incurring relatively small
> regressions (relative to the workload and relative to the variation).
> 
> The following are a few synthetic benchmarks:
> 
> Test 1: Small machine (30G RAM, 36 CPUs)
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
>
> ...
>
> @@ -1267,12 +1270,22 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>  
>  			/* must delete to avoid corrupting pcp list */
>  			list_del(&page->pcp_list);
> +			batch -= nr_pages;
>  			count -= nr_pages;
>  			pcp->count -= nr_pages;
>  
>  			__free_one_page(page, pfn, zone, order, mt, FPI_NONE);
>  			trace_mm_page_pcpu_drain(page, order, mt);
> -		} while (count > 0 && !list_empty(list));
> +		} while (batch > 0 && !list_empty(list));
> +
> +		/*
> +		 * Prevent starving the lock for other users; every pcp->batch
> +		 * pages freed, relinquish the zone lock if it is contended.
> +		 */
> +		if (count && spin_is_contended(&zone->lock)) {
> +			spin_unlock_irqrestore(&zone->lock, flags);
> +			spin_lock_irqsave(&zone->lock, flags);
> +		}
>  	}

Pretty this isn't.

Sigh, we do so much stuff here and in __free_one_page().

What sort of guarantee do we have that the contending task will be able
to get in and grab the spinlock in that tiny time window?