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@xxxxxxx>
> >>
> >> correct lazy_max_pages() return value if the number of online
> >> CPUs is power of 2
> >>
> >> Signed-off-by: zijun_hu <zijun_hu@xxxxxxx>
> >> ---
> >> 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.
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