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

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Date:   Fri, 30 Mar 2018 09:39:03 +0800
From:   Ganesh Mahendran <opensource.ganesh@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Joe Perches <joe@...ches.com>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v5] ANDROID: binder: change down_write to down_read

2018-03-30 9:29 GMT+08:00 Minchan Kim <minchan@...nel.org>:
> 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"?

After applying this patch in our devices, app launch time increases
about 15% in average.
"This patch makes performance regression", yes, from the results, it
is like this.

I will do more test of this patch.

>
>>
>> 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.
>
> Thanks.