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

[高翔 <gaoxiang17@xiaomi.com>]

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From this perspective, it seems better to add it to the trace. Then shall I make a patch to add to the trace?


________________________________
发件人: David Hildenbrand <david@redhat.com>
发送时间: 2025年8月16日 15:49:52
收件人: Andrew Morton
抄送: Xiang Gao; lorenzo.stoakes@oracle.com; Liam.Howlett@oracle.com; vbabka@suse.cz; rppt@kernel.org; surenb@google.com; mhocko@suse.com; linux-mm@kvack.org; linux-kernel@vger.kernel.org; 高翔
主题: [External Mail]Re: [PATCH] mm/cma: print total and used pages in cma_alloc()

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On 16.08.25 09:34, Andrew Morton wrote:
> On Sat, 16 Aug 2025 08:56:47 +0200 David Hildenbrand <david@redhat.com> wrote:
>
>> On 16.08.25 08:45, Andrew Morton wrote:
>>> On Sat, 16 Aug 2025 08:27:39 +0200 David Hildenbrand <david@redhat.com> 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


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