Re: [PATCH] mm/cma: print total and used pages in cma_alloc()

[David Hildenbrand]

On 16.08.25 09:34, Andrew Morton wrote:
> On Sat, 16 Aug 2025 08:56:47 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > On 16.08.25 08:45, Andrew Morton wrote:
> > > On Sat, 16 Aug 2025 08:27:39 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >
> > > > > @@ -858,8 +869,8 @@ static struct page *__cma_alloc(struct cma *cma, unsigned long count,
> > > > >     	if (!cma || !cma->count)
> > > > >     		return page;
> > > > >     
> > > > > -	pr_debug("%s(cma %p, name: %s, count %lu, align %d)\n", __func__,
> > > > > -		(void *)cma, cma->name, count, align);
> > > > > +	pr_debug("%s(cma %p, name: %s, total pages: %lu, used pages: %lu, request pages: %lu, align %d)\n",
> > > > > +		__func__, (void *)cma, cma->name, cma->count, cma_get_used_pages(cma), count, align);
> > > >
> > > > 		^ one space missing for proper indentation.
> > > >
> > > > But doing another spinlock cycle just for debugging purposes? That does
> > > > not feel right, sorry.
> > >
> > > If we're calling pr_debug() frequently enough for this to matter, we
> > > have other problems!
> >
> > We call it for each and every actual CMA allocation? I really don't see
> > why we want to just randomly make CMA allocation latency worse.
>
> pr_debug() is 12 million times more expensive than a spin_lock()!
>
> > Is the existing pr_debug() a problem? Maybe. But who actually has debug
> > messages enabled in any sane setup?
>
> Nobody, clearly.  If anyone enabled pr_debug() in here, they'd
> immediately have to remove those statements to get any work done.  Kill
> it.

I just learned that pr_debug() on a !CONFIG_DEBUG kernel translates to 
no_printk(), which is just a mostly-empty macro that doesn't really use 
any of the parameters.

I would assume the cma_get_used_pages() would get completely optimized 
out in that case.

So, I don't care, but ... moving to tracing seems much more reasonable.

--
Cheers

David / dhildenb