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

[Minchan Kim]

On Fri, Mar 30, 2018 at 09:39:03AM +0800, Ganesh Mahendran wrote:
> 2018-03-30 9:29 GMT+08:00 Minchan Kim <minchan@xxxxxxxxxx>:
> > 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"?
> 
> 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.

Thanks for the testing. Every apps increases time?
I will appreciate if you confirm the problem again.

I guess the reason is currently rw_semaphore's preferrence for writer
at up_write so If binder uses down_read instead of down_write, binder
should compete other down_read callers at wake up time from up_write
and could lose wakeup chance if it is low priority. 

Anyway, I think it's abuse of using down_write to get lock holding
asap where it's down_read enough. I guess there is tradeoff based on
binder's priority. 
If we go to the speculative page faults or range-lock instad of mmap_sem,
the situation would be much improved.

Anyway, currently, I have no idea to overcome such prioriy inversion
problem from rw_semaphore and I believe generally changing from
down_read to down_write is good thing to prevent contention.

Ccing maintainers.