linux-kernel - Re: [PATCH v5] ANDROID: binder: change down_write to down

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Message-ID: <CADAEsF8_vptm7p2y2ekj8m6t=bz4pwC1+np7r2-YnAKbnEsA4w@mail.gmail.com>
Date:   Mon, 2 Apr 2018 14:46:14 +0800
From:   Ganesh Mahendran <opensource.ganesh@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joe Perches <joe@...ches.com>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v5] ANDROID: binder: change down_write to down_read

2018-04-02 14:34 GMT+08:00 Minchan Kim <minchan@...nel.org>:
> 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@...nel.org>:
>> > > > 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.

I will use binderThroghput tool to test the patch today or tomorrow.

Thanks.

> Please let me know how you measure without noise so I'd like to reproduce
> the result in my phone.
>
> I will do binderThroghput test, too.
>
>