Re: [PATCH 4/4] mm/slub: refactor calculate_order() and calc_slab_order()

From: Baoquan He <bhe@redhat.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: David Rientjes <rientjes@google.com>,
	Christoph Lameter <cl@linux.com>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>,
	Jay Patel <jaypatel@linux.ibm.com>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Pekka Enberg <penberg@kernel.org>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	linux-mm@kvack.org, patches@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/4] mm/slub: refactor calculate_order() and calc_slab_order()
Date: Fri, 22 Sep 2023 15:29:14 +0800	[thread overview]
Message-ID: <ZQ1CSgj5+Dy/rWEN@MiWiFi-R3L-srv> (raw)
In-Reply-To: <4662588e-fc8b-1854-57f8-d15e08a3c368@suse.cz>

On 09/22/23 at 09:00am, Vlastimil Babka wrote:
> On 9/16/23 03:28, Baoquan He wrote:
> > On 09/08/23 at 04:53pm, Vlastimil Babka wrote:
> >> @@ -4152,7 +4147,7 @@ static inline int calculate_order(unsigned int size)
> >>  		 * order on systems that appear larger than they are, and too
> >>  		 * low order on systems that appear smaller than they are.
> >>  		 */
> >> -		nr_cpus = num_present_cpus();
> >> +		unsigned int nr_cpus = num_present_cpus();
> >>  		if (nr_cpus <= 1)
> >>  			nr_cpus = nr_cpu_ids;
> >>  		min_objects = 4 * (fls(nr_cpus) + 1);
> > 
> > A minor concern, should we change 'min_objects' to be a local static
> > to avoid the "if (!min_objects) {" code block every time?  It's deducing
> > the value from nr_cpus, we may not need do the calculation each time.
> 
> Maybe, although it's not a hot path. But we should make sure the
> num_present_cpus() cannot change. Could it be e.g. low (1) very early when
> we bootstrap the initial caches, but then update and at least most of the
> caches then reflect the real number of cpus? With a static we would create
> everything with 1.

Yeah, I was silly, didn't think about it. We may check via system_state,
but it's not worth to bother since it's not hot path as you said. Sorry for
the noise.