linux-kernel - Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max

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Date:   Wed, 21 Sep 2016 17:35:24 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     zijun_hu <zijun_hu@...o.com>
cc:     zijun_hu@....com, Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org, tj@...nel.org,
        mingo@...nel.org, iamjoonsoo.kim@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max_pages() return
 value

On Thu, 22 Sep 2016, zijun_hu wrote:

> On 2016/9/22 5:21, David Rientjes wrote:
> > On Wed, 21 Sep 2016, zijun_hu wrote:
> > 
> >> From: zijun_hu <zijun_hu@....com>
> >>
> >> correct lazy_max_pages() return value if the number of online
> >> CPUs is power of 2
> >>
> >> Signed-off-by: zijun_hu <zijun_hu@....com>
> >> ---
> >>  mm/vmalloc.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> >> index a125ae8..2804224 100644
> >> --- a/mm/vmalloc.c
> >> +++ b/mm/vmalloc.c
> >> @@ -594,7 +594,9 @@ static unsigned long lazy_max_pages(void)
> >>  {
> >>  	unsigned int log;
> >>  
> >> -	log = fls(num_online_cpus());
> >> +	log = num_online_cpus();
> >> +	if (log > 1)
> >> +		log = (unsigned int)get_count_order(log);
> >>  
> >>  	return log * (32UL * 1024 * 1024 / PAGE_SIZE);
> >>  }
> > 
> > The implementation of lazy_max_pages() is somewhat arbitrarily defined, 
> > the existing approximation has been around for eight years and 
> > num_online_cpus() isn't intended to be rounded up to the next power of 2.  
> > I'd be inclined to just leave it as it is.
> > 
> do i understand the intent in current code logic as below ?
> [8, 15) roundup to 16?
> [32, 63) roundup to 64?
> 

The intent is as it is implemented; with your change, lazy_max_pages() is 
potentially increased depending on the number of online cpus.  This is 
only a heuristic, changing it would need justification on why the new 
value is better.  It is opposite to what the comment says: "to be 
conservative and not introduce a big latency on huge systems, so go with
a less aggressive log scale."  NACK to the patch.