Re: [PATCH v2 0/2] Try to release mmap_lock temporarily in smaps_rollup

[Michel Lespinasse]

On Thu, Aug 13, 2020 at 9:11 AM Chinwen Chang
<chinwen.chang@xxxxxxxxxxxx> wrote:
> On Thu, 2020-08-13 at 02:53 -0700, Michel Lespinasse wrote:
> > On Wed, Aug 12, 2020 at 7:14 PM Chinwen Chang
> > <chinwen.chang@xxxxxxxxxxxx> wrote:
> > > Recently, we have observed some janky issues caused by unpleasantly long
> > > contention on mmap_lock which is held by smaps_rollup when probing large
> > > processes. To address the problem, we let smaps_rollup detect if anyone
> > > wants to acquire mmap_lock for write attempts. If yes, just release the
> > > lock temporarily to ease the contention.
> > >
> > > smaps_rollup is a procfs interface which allows users to summarize the
> > > process's memory usage without the overhead of seq_* calls. Android uses it
> > > to sample the memory usage of various processes to balance its memory pool
> > > sizes. If no one wants to take the lock for write requests, smaps_rollup
> > > with this patch will behave like the original one.
> > >
> > > Although there are on-going mmap_lock optimizations like range-based locks,
> > > the lock applied to smaps_rollup would be the coarse one, which is hard to
> > > avoid the occurrence of aforementioned issues. So the detection and
> > > temporary release for write attempts on mmap_lock in smaps_rollup is still
> > > necessary.
> >
> > I do not mind extending the mmap lock API as needed. However, in the
> > past I have tried adding rwsem_is_contended to mlock(), and later to
> > mm_populate() paths, and IIRC gotten pushback on it both times. I
> > don't feel strongly on this, but would prefer if someone else approved
> > the rwsem_is_contended() use case.
> >
> Hi Michel,
>
> Thank you for your kind feedback.
>
> In my opinion, the difference between the case in smaps_rollup and the
> one in your example is that, for the former, the interference comes from
> the outside of the affected process, for the latter, it doesn't.
>
> In other words, anyone may use smaps_rollup to probe the affected
> process without the information about what step the affected one is
> executing.

Thanks, that is a good enough point for me :)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.