Re: [PATCH v4 4/4] mm: memcg: use non-unified stats flushing for userspace reads

Wei Xu
2023-09-11T20:52:02Z

On Mon, Sep 11, 2023 at 12:34 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Mon 11-09-23 12:15:24, Wei Xu wrote:
> > On Mon, Sep 11, 2023 at 6:11 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 07-09-23 17:52:12, Wei Xu wrote:
> > > [...]
> > > > I tested this patch on a machine with 384 CPUs using a microbenchmark
> > > > that spawns 10K threads, each reading its memory.stat every 100
> > > > milliseconds.
> > >
> > > This is rather extreme case but I wouldn't call it utterly insane
> > > though.
> > >
> > > > Most of memory.stat reads take 5ms-10ms in kernel, with
> > > > ~5% reads even exceeding 1 second.
> > >
> > > Just curious, what would numbers look like if the mutex is removed and
> > > those threads would be condending on the existing spinlock with lock
> > > dropping in place and removed. Would you be willing to give it a shot?
> >
> > Without the mutex and with the spinlock only, the common read latency
> > of memory.stat is still 5ms-10ms in kernel. There are very few reads
> > (<0.003%) going above 10ms and none more than 1 second.
>
> Is this with the existing spinlock dropping and same 10k potentially
> contending readers?

Yes, it is the same test (10K contending readers). The kernel change
is to remove stats_user_flush_mutex from mem_cgroup_user_flush_stats()
so that the concurrent mem_cgroup_user_flush_stats() requests directly
contend on cgroup_rstat_lock in cgroup_rstat_flush().

> --
> Michal Hocko
> SUSE Labs