Re: [PATCH] proc: Avoid a thundering herd of threads freeing proc dentries

[Junxiao Bi]

On 6/20/20 9:27 AM, Matthew Wilcox wrote:

> On Fri, Jun 19, 2020 at 05:42:45PM -0500, Eric W. Biederman wrote:
> > Junxiao Bi <junxiao.bi@xxxxxxxxxx> writes:
> > > Still high lock contention. Collect the following hot path.
> > A different location this time.
> >
> > I know of at least exit_signal and exit_notify that take thread wide
> > locks, and it looks like exit_mm is another.  Those don't use the same
> > locks as flushing proc.
> >
> >
> > So I think you are simply seeing a result of the thundering herd of
> > threads shutting down at once.  Given that thread shutdown is fundamentally
> > a slow path there is only so much that can be done.
> >
> > If you are up for a project to working through this thundering herd I
> > expect I can help some.  It will be a long process of cleaning up
> > the entire thread exit process with an eye to performance.
> Wengang had some tests which produced wall-clock values for this problem,
> which I agree is more informative.
>
> I'm not entirely sure what the customer workload is that requires a
> highly threaded workload to also shut down quickly.  To my mind, an
> overall workload is normally composed of highly-threaded tasks that run
> for a long time and only shut down rarely (thus performance of shutdown
> is not important) and single-threaded tasks that run for a short time.

The real workload is a Java application working in server-agent mode, 
issue happened in agent side, all it do is waiting works dispatching 
from server and execute. To execute one work, agent will start lots of 
short live threads, there could be a lot of threads exit same time if 
there were a lots of work to execute, the contention on the exit path 
caused a high %sys time which impacted other workload.

Thanks,

Junxiao.

> Understanding this workload is important to my next suggestion, which
> is that rather than searching for all the places in the exit path which
> contend on a single spinlock, we simply set the allowed CPUs for an
> exiting task to include only the CPU that this thread is running on.
> It will probably run faster to take the threads down in series on one
> CPU rather than take them down in parallel across many CPUs (or am I
> mistaken?  Is there inherently a lot of parallelism in the thread
> exiting process?)