Re: d28296d248: stress-ng.sigsegv.ops_per_sec -82.7% regression

[Eric W. Biederman]

Alexey Gladkov <gladkov.alexey@xxxxxxxxx> writes:

> On Wed, Feb 24, 2021 at 10:54:17AM -0600, Eric W. Biederman wrote:
>> kernel test robot <oliver.sang@xxxxxxxxx> writes:
>> 
>> > Greeting,
>> >
>> > FYI, we noticed a -82.7% regression of stress-ng.sigsegv.ops_per_sec due to commit:
>> >
>> >
>> > commit: d28296d2484fa11e94dff65e93eb25802a443d47 ("[PATCH v7 5/7] Reimplement RLIMIT_SIGPENDING on top of ucounts")
>> > url: https://github.com/0day-ci/linux/commits/Alexey-Gladkov/Count-rlimits-in-each-user-namespace/20210222-175836
>> > base: https://git.kernel.org/cgit/linux/kernel/git/shuah/linux-kselftest.git next
>> >
>> > in testcase: stress-ng
>> > on test machine: 48 threads Intel(R) Xeon(R) CPU E5-2697 v2 @ 2.70GHz with 112G memory
>> > with following parameters:
>> >
>> > 	nr_threads: 100%
>> > 	disk: 1HDD
>> > 	testtime: 60s
>> > 	class: interrupt
>> > 	test: sigsegv
>> > 	cpufreq_governor: performance
>> > 	ucode: 0x42e
>> >
>> >
>> > In addition to that, the commit also has significant impact on the
>> > following tests:
>> 
>> Thank you.  Now we have a sense of where we need to test the performance
>> of these changes carefully.
>
> One of the reasons for this is that I rolled back the patch that changed
> the ucounts.count type to atomic_t. Now get_ucounts() is forced to use a
> spin_lock to increase the reference count.

Which given the hickups with getting a working version seems justified.

Now we can add incremental patches on top to improve the performance.


Eric