linux-kernel - Re: [PATCH 4/4] mm/slub: refactor calculate_order() and calc_slab

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Message-ID: <ZQ1CSgj5+Dy/rWEN@MiWiFi-R3L-srv>
Date:   Fri, 22 Sep 2023 15:29:14 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Jay Patel <jaypatel@...ux.ibm.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] mm/slub: refactor calculate_order() and
 calc_slab_order()

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.