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

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Sun, Jun 21, 2020 at 10:15:39PM -0700, Junxiao Bi wrote:
> 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.

How about this for a micro?  Executes in about ten seconds on my laptop.
You might need to tweak it a bit to get better timing on a server.

// gcc -pthread -O2 -g -W -Wall
#include <pthread.h>
#include <unistd.h>

void *worker(void *arg)
{
	int i = 0;
	int *p = arg;

	for (;;) {
		while (i < 1000 * 1000) {
			i += *p;
		}
		sleep(1);
	}
}

int main(int argc, char **argv)
{
	pthread_t threads[20][100];
	int i, j, one = 1;

	for (i = 0; i < 1000; i++) {
		for (j = 0; j < 100; j++)
			pthread_create(&threads[i % 20][j], NULL, worker, &one);
		if (i < 5)
			continue;
		for (j = 0; j < 100; j++)
			pthread_cancel(threads[(i - 5) %20][j]);
	}

	return 0;
}