linux-kernel - Re: [PATCH v2] mm/cma: print total and used count in cma

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Message-ID: <f8dde346-8a81-4cca-8497-987f6e4b5e58@redhat.com>
Date: Mon, 30 Sep 2024 11:34:09 +0200
From: David Hildenbrand <david@...hat.com>
To: Xiang Gao <gxxa03070307@...il.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 gaoxiang17 <gaoxiang17@...omi.com>
Subject: Re: [PATCH v2] mm/cma: print total and used count in cma_alloc()

On 29.09.24 05:27, Xiang Gao wrote:
> From: gaoxiang17 <gaoxiang17@...omi.com>
> 

We should add here

"To debug CMA allocations (especially failing ones), it is valuable to 
know the state of CMA: how many pages out of the total ones are 
allocated, and how many were requested to be allocated. Let's print
some more information."

I assume Andrew can fix that up when applying.

> before:
> [   24.407814] cma: cma_alloc(cma (____ptrval____), name: reserved, count 1, align 0)
> [   24.413397] cma: cma_alloc(cma (____ptrval____), name: reserved, count 1, align 0)
> [   24.415886] cma: cma_alloc(cma (____ptrval____), name: reserved, count 1, align 0)
> 
> after:
> [   24.069738] cma: cma_alloc(cma (____ptrval____), name: reserved, total pages: 16384, used pages: 64, request pages: 1, align 0)
> [   24.075317] cma: cma_alloc(cma (____ptrval____), name: reserved, total pages: 16384, used pages: 65, request pages: 1, align 0)
> [   24.078455] cma: cma_alloc(cma (____ptrval____), name: reserved, total pages: 16384, used pages: 66, request pages: 1, align 0)
> 
> Signed-off-by: gaoxiang17 <gaoxiang17@...omi.com>
> ---
>   mm/cma.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index 2d9fae939283..90b3fdbac19c 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -403,6 +403,17 @@ static void cma_debug_show_areas(struct cma *cma)
>   	spin_unlock_irq(&cma->lock);
>   }
>   
> +static unsigned long cma_get_used_pages(struct cma *cma)
> +{
> +	unsigned long used;
> +
> +	spin_lock_irq(&cma->lock);
> +	used = bitmap_weight(cma->bitmap, (int)cma_bitmap_maxno(cma));
> +	spin_unlock_irq(&cma->lock);

This adds overhead to each allocation, even if debug outputs are ignored 
I assume?

I wonder if we'd want to print these details only when our allocation 
failed?

Alternatively, we could actually track how many pages are allocated in 
the cma, so we don't have to traverse the complete bitmap on every 
allocation.


-- 
Cheers,

David / dhildenb