On Sat, 14 Jan 2023 13:00:00 -0800 Nathan Huckleberry <nhuck@xxxxxxxxxx>
> On Fri, Jan 13, 2023 at 6:20 PM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
> >
> > Hi Nathan!
> >
> > On 2023/1/14 05:07, Nathan Huckleberry wrote:
> > > Add a WQ flag that allows workqueues to use SCHED_FIFO with the least
> > > imporant RT priority. This can reduce scheduler latency for IO
> > > post-processing when the CPU is under load without impacting other RT
> > > workloads. This has been shown to improve app startup time on Android
> > > [1].
> >
> > Thank you all for your effort on this. Unfortunately I have no time to
> > setup the test [1] until now. If it can be addressed as a new workqueue
> > feature, that would be much helpful to me. Otherwise, I still need to
> > find a way to resolve the latest Android + EROFS latency problem.
> >
>
> The above patch and following diff should have equivalent performance
> to [1], but I have not tested it.
>
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index ccf7c55d477f..a9c3893ad1d4 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -201,7 +201,7 @@ static inline int z_erofs_init_workqueue(void)
> * scheduling overhead, perhaps per-CPU threads should be better?
> */
> z_erofs_workqueue = alloc_workqueue("erofs_unzipd",
> - WQ_UNBOUND | WQ_HIGHPRI,
> + WQ_SCHED_FIFO,
> onlinecpus + onlinecpus / 4);
> return z_erofs_workqueue ? 0 : -ENOMEM;
>
The chart in [1] shows the change in scheduling latency that is the latency
between when the task (workqueue kworker vs kthread_worker) became eligible
to run to when it actually started running.
+-------------------------+-----------+----------------+---------+
| | workqueue | kthread_worker | diff |
+-------------------------+-----------+----------------+---------+
| Average (us) | 15253 | 2914 | -80.89% |
| Median (us) | 14001 | 2912 | -79.20% |
| Minimum (us) | 3117 | 1027 | -67.05% |
| Maximum (us) | 30170 | 3805 | -87.39% |
| Standard deviation (us) | 7166 | 359 | |
+-------------------------+-----------+----------------+---------+
The change in scheduling latency after this one-line patch for
workqueue workers like kblockd_workqueue and rcu_gp_wq is welcome.
> > >
> > > [1]:
> > > https://lore.kernel.org/linux-erofs/20230106073502.4017276-1-dhavale@xxxxxxxxxx/
> > > [2]:
> > > https://lore.kernel.org/linux-f2fs-devel/20220802192437.1895492-1-daeho43@xxxxxxxxx/
> > > [3]:
> > > https://lore.kernel.org/dm-devel/20220722093823.4158756-4-nhuck@xxxxxxxxxx/
> > > [4]:
> > > https://lore.kernel.org/dm-crypt/20200706173731.3734-1-ignat@xxxxxxxxxxxxxx/
