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

From: Minchan Kim <minchan@kernel.org>
To: Ganesh Mahendran <opensource.ganesh@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	"Joe Perches" <joe@perches.com>,
	"Arve Hjønnevåg" <arve@android.com>,
	"Todd Kjos" <tkjos@google.com>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Martijn Coenen" <maco@android.com>
Subject: Re: [PATCH v5] ANDROID: binder: change down_write to down_read
Date: Fri, 30 Mar 2018 10:29:21 +0900	[thread overview]
Message-ID: <20180330012921.GB255979@rodete-desktop-imager.corp.google.com> (raw)
In-Reply-To: <CADAEsF8Y2CJsuom=9au0-uvZAALvrLTdTnrkxrGjW9C9F6hTVg@mail.gmail.com>

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@kernel.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.

Thanks.