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[2003:d8:2f23:c700:d8ae:41bf:492a:9e4c]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-45a1fe2c019sm66592525e9.17.2025.08.16.00.49.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 16 Aug 2025 00:49:53 -0700 (PDT) Message-ID: <167acb82-1368-4c8e-89bd-8dbe4877d5bb@redhat.com> Date: Sat, 16 Aug 2025 09:49:52 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/cma: print total and used pages in cma_alloc() To: Andrew Morton Cc: 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, gaoxiang17 References: <20250816042842.3959315-1-gxxa03070307@gmail.com> <20250815234528.882ab58247cefc96e4137811@linux-foundation.org> <701bfbb4-6c12-4614-a322-def3ca923c78@redhat.com> <20250816003418.b2a62f6ddbcf9468fde87a18@linux-foundation.org> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 16.08.25 09:34, Andrew Morton wrote: > On Sat, 16 Aug 2025 08:56:47 +0200 David Hildenbrand wrote: > >> On 16.08.25 08:45, Andrew Morton wrote: >>> On Sat, 16 Aug 2025 08:27:39 +0200 David Hildenbrand 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