On 11/5/18 9:50 AM, Linus Torvalds wrote:
On Sun, Nov 4, 2018 at 9:08 PM kernel test robot <[email protected]> wrote:
FYI, we noticed a -64.1% regression of will-it-scale.per_thread_ops
due to commit 9bc8039e715d ("mm: brk: downgrade mmap_sem to read when
shrinking")
Ugh. That looks pretty bad.
in testcase: will-it-scale
on test machine: 8 threads Ivy Bridge with 16G memory
with following parameters:
nr_task: 100%
mode: thread
test: brk1
ucode: 0x20
cpufreq_governor: performance
The reason seems to be way more scheduler time due to lots more
context switches:
34925294 ± 18% +270.3% 1.293e+08 ± 4%
will-it-scale.time.voluntary_context_switches
Yang Shi, would you mind taking a look at what's going on?
No problem.
Actually, the commit is mainly for optimizing the long stall time caused
by holding mmap_sem by write when unmapping or shrinking large mapping.
It downgrades write mmap_sem to read when zapping pages. So, it looks
the downgrade incurs more context switches. This is kind of expected.
However, the test looks just shrink the mapping with one normal 4K page
size. It sounds the overhead of context switches outpace the gain in
this case at the first glance.
Since the optimization makes more sense to large mapping, how about
restore the mapping size limit, e.g. just downgrade mmap_sem for >= 1g
(PUD_SIZE) mapping?
Thanks,
Yang
Linus
