Re: [PATCH v5] ANDROID: binder: change down_write to down_read

[Minchan Kim]

Hi Ganesh,

Isn't there any update?

On Mon, Apr 2, 2018 at 7:32 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> Hi Ganesh,
>
> On Mon, Apr 02, 2018 at 06:01:59PM +0800, Ganesh Mahendran wrote:
>> 2018-04-02 15:11 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>:
>> > On Mon, Apr 02, 2018 at 02:46:14PM +0800, Ganesh Mahendran wrote:
>> >> 2018-04-02 14:34 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>:
>> >> > On Fri, Mar 30, 2018 at 12:04:07PM +0200, Greg Kroah-Hartman wrote:
>> >> >> On Fri, Mar 30, 2018 at 10:29:21AM +0900, Minchan Kim wrote:
>> >> >> > Hi Ganesh,
>> >> >> >
>> >> >> > On Fri, Mar 30, 2018 at 09:21:55AM +0800, Ganesh Mahendran wrote:
>> >> >> > > 2018-03-29 14:54 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>:
>> >> >> > > > binder_update_page_range needs down_write of mmap_sem because
>> >> >> > > > vm_insert_page need to change vma->vm_flags to VM_MIXEDMAP unless
>> >> >> > > > it is set. However, when I profile binder working, it seems
>> >> >> > > > every binder buffers should be mapped in advance by binder_mmap.
>> >> >> > > > It means we could set VM_MIXEDMAP in binder_mmap time which is
>> >> >> > > > already hold a mmap_sem as down_write so binder_update_page_range
>> >> >> > > > doesn't need to hold a mmap_sem as down_write.
>> >> >> > > >
>> >> >> > > > Android suffers from mmap_sem contention so let's reduce mmap_sem
>> >> >> > > > down_write.
>> >> >> > >
>> >> >> > > Hi, Minchan:
>> >> >> > >
>> >> >> > > It seems there is performance regression of this patch.
>> >> >> >
>> >> >> > You mean "This patch aims for solving performance regression" not "This patch
>> >> >> > makes performance regression"?
>> >> >> >
>> >> >> > >
>> >> >> > > Do you have some test result of android app launch time or binderThroughput?
>> >> >> >
>> >> >> > Unfortunately, I don't have any number. The goal is to reduce the number of
>> >> >> > call mmap_sem as write-side lock because it makes priority inversion of threads
>> >> >> > easily and that's one of clear part I spot that we don't need write-side lock.
>> >> >>
>> >> >> Please always run the binderThroughput tests when making binder changes
>> >> >> (there is a binder test suite in the CTS Android tests), as that ensures
>> >> >> that you are not causing performance regressions as well as just normal
>> >> >> bug regressions :)
>> >> >
>> >> > Thanks for the information. I didn't notice that such kinds of tests for
>> >> > binder. I will keep it in mind.
>> >> >
>> >> > Today, I have setup the testing for my phone and found testing was very
>> >> > fluctuating even without my patch. It might be not good with my test
>> >> > skill. I emulated user's behavior with various touch event. With it, I open
>> >> > various apps and play with them several times. Before starting the test,
>> >> > I did "adb shell stop && adb shell start && echo 3 > /proc/sys/vm/drop_caches"
>> >> >
>> >> > Such 15% noise was very easy to make it.
>> >> >
>> >> > Ganesh, How did you measure? What's the stddev?
>> >>
>> >> Hi, Minchan:
>> >>
>> >> Sorry for the late response, a little busy these days. :)
>> >>
>> >> We have our own test tools to measure app launch time, or you can use
>> >> android systrace to get the app launch time. We tested your V1 patch:
>> >> https://patchwork.kernel.org/patch/10312057/
>> >> and found app lunch time regression.
>> >
>> > V1 had a bug with VM_MAYWRITE. Could you confirm it with v5?
>>
>> I have finished binder Throughput test. The test result is stable,
>> there is no performance
>> regression found both in v1 and v5.
>
> Thanks for the test! Now I'm struggling with setting up BinderThrough test.
> Binder matainers:
> If it's really one every binder contributors should do before the
> sending their patch, couldn't we have them in kernel directory like kselftest?
> Like me who understand just a part of code, it's hard to download android
> userspace full code and build/test.
>
>
>>
>>             base            patch_v1         patch_v5
>> -----------------------------------------------------------
>>             91223.4      90560.2           89644.5
>>             90520.3      89583.1           89048.2
>>             89833.2      90247.6           90091.3
>>             90740.2      90276.7           90994.2
>>             89703.5      90112.4           89994.6
>>             89945.1      89122.8           88937.7
>>             89872.8      90357.3           89307.4
>>             89913.2      90355.4           89563.8
>>             88979         90393.4           90182.8
>>             89577.3      90946.8           90441.4
>> AVG    90030.8      90195.57          89820.59
>
> Yes, no regression.
>
>>
>> Before the test, I stop the android framework by:
>>     adb shell stop
>>
>> >
>> > Please tell me more detail. What apps are slower compared to old?
>> > Every apps are slowed with avg 15%? Then, what's the stddev?
>>
>> Not all of the apps slowed 15%, The app *avg* launch time slowed 15%.
>> And We will re-launch the test tomorrow: base, v1,v5. We will get the
>> test result in two days later. Then I will post all the app launch time details.
>
> I'm also trying to make stable result in my side but it's really hard to
> get. Please post stddev of each app as well as avg when you finished testing.
> I really appreicate you.
>
>>
>> >
>> > The reason I'm asking is as I mentioned, it would be caused by rw_semaphore
>> > implementation and priority of threads which calls binder operation so I
>> > guess it would be not deterministic.
>> >
>> > When I had an simple experiment, it was very fluctuating as I expected.
>> > (the testing enviroment might be not good in my side).
>> > If it's real problem on real practice, better fix is not using write_lock
>> > of mmap_sem(it's abusing the write-side lock)  but should adjust priority,
>> > I think. What do you think?
>>
>> If you want to narrow the range of the problem. We can disable binder priority
>> inherit, and do not set the priority(currently it is nice -10 or fifo)
>> of top app in Android AMS.
>> I think we need to wait for the test result to see whether it really
>> has performance
>> regression.
>
> Look at up_write.
>
> (Let's assume process B is head of wait list of rw_semaphore, and then C, D, E)
> If the process B is trying to down_write and previous lock holder A is
> called up_write, the only B could be waked up so there is no contention
> to get CPU slice. It's the current as-is but if we changes B to try to
> down_read instead of down_write, B should be competed with other down_read
> C,D,E in so the chance would be rare to be scheduled.
>
> It's really (timing|priority of binder and other threads) problem so I don't
> understand what you said how we could narrow down the problem with disabling
> binder priority.



-- 
Kind regards,
Minchan Kim