[PATCH 0/2] Add epoll round robin wakeup mode

[Jason Baron]

Hi,

When we are sharing a wakeup source among multiple epoll fds, we end up with
thundering herd wakeups, since there is currently no way to add to the
wakeup source exclusively. This series introduces 2 new epoll flags,
EPOLLEXCLUSIVE for adding to a wakeup source exclusively. And EPOLLROUNDROBIN
which is to be used in conjunction to EPOLLEXCLUSIVE to evenly
distribute the wakeups. I'm showing perf results from the simple pipe() usecase
below. But this patch was originally motivated by a desire to improve
wakeup balance and cpu usage for a shared listen socket().

Perf stat, 3.19.0-rc7+, 4 core, Intel(R) Xeon(R) CPU E3-1265L v3 @ 2.50GHz:

pipe test wake all:

 Performance counter stats for './wake':

      10837.480396      task-clock (msec)         #    1.879 CPUs utilized          
           2047108      context-switches          #    0.189 M/sec                  
            214491      cpu-migrations            #    0.020 M/sec                  
               247      page-faults               #    0.023 K/sec                  
       23655687888      cycles                    #    2.183 GHz                    
   <not supported>      stalled-cycles-frontend  
   <not supported>      stalled-cycles-backend   
       11242141621      instructions              #    0.48  insns per cycle        
        2313479486      branches                  #  213.470 M/sec                  
          13679036      branch-misses             #    0.59% of all branches        

       5.768295821 seconds time elapsed

pipe test wake balanced:

 Performance counter stats for './wake -o':

        291.250312      task-clock (msec)         #    0.094 CPUs utilized          
             40308      context-switches          #    0.138 M/sec                  
              1448      cpu-migrations            #    0.005 M/sec                  
               248      page-faults               #    0.852 K/sec                  
         646407197      cycles                    #    2.219 GHz                    
   <not supported>      stalled-cycles-frontend  
   <not supported>      stalled-cycles-backend   
         364256883      instructions              #    0.56  insns per cycle        
          65775397      branches                  #  225.838 M/sec                  
            535637      branch-misses             #    0.81% of all branches        

       3.086694452 seconds time elapsed

Rough epoll manpage text:

EPOLLEXCLUSIVE
	Provides exclusive wakeups when attaching multiple epoll fds to a
	shared wakeup source. Must be specified on an EPOLL_CTL_ADD operation.

EPOLLROUNDROBIN
	Provides balancing for exclusive wakeups when attaching multiple epoll
	fds to a shared wakeup soruce. Must be specificed with EPOLLEXCLUSIVE
	during an EPOLL_CTL_ADD operation.


Thanks,

-Jason

#include <unistd.h>
#include <sys/epoll.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define NUM_THREADS 100
#define NUM_EVENTS 20000
#define EPOLLEXCLUSIVE (1 << 28)
#define EPOLLBALANCED (1 << 27)

int optimize, exclusive;
int p[2];
pthread_t threads[NUM_THREADS];
int event_count[NUM_THREADS];

struct epoll_event evt = {
	.events = EPOLLIN 
};

void die(const char *msg) {
    perror(msg);
    exit(-1);
}

void *run_func(void *ptr)
{
	int i = 0;
	int j = 0;
	int ret;
	int epfd;
	char buf[4];
	int id = *(int *)ptr;
	int *contents;

	if ((epfd = epoll_create(1)) < 0)
		die("create");

	if (optimize)
		evt.events |= ((EPOLLBALANCED | EPOLLEXCLUSIVE));
	else if (exclusive)
		evt.events |= EPOLLEXCLUSIVE;
	ret = epoll_ctl(epfd, EPOLL_CTL_ADD, p[0], &evt);
	if (ret)
		perror("epoll_ctl add error!\n");

	while (1) { 
    		ret = epoll_wait(epfd, &evt, 10000, -1);
		ret = read(p[0], buf, sizeof(int));
		if (ret == 4)
			event_count[id]++;
	}
}

int main(int argc, char *argv[])
{
	int ret, i, j;
	int id[NUM_THREADS];
	int total = 0;
	int nohit = 0;
	int extra_wakeups = 0;

	if (argc == 2) {
		if (strcmp(argv[1], "-o") == 0)
			optimize = 1;
		if (strcmp(argv[1], "-e") == 0)
			exclusive = 1;
	}

	if (pipe(p) < 0)
		die("pipe");

	for (i = 0; i < NUM_THREADS; i++) {
		id[i] = i;
		pthread_create(&threads[i], NULL, run_func, &id[i]);
	} 

	for (j = 0; j < NUM_EVENTS; j++) {
		write(p[1], p, sizeof(int));
		usleep(100);
	}

	for (i = 0; i < NUM_THREADS; i++) {
		pthread_cancel(threads[i]);
		printf("joined: %d\n", i);
		printf("event count: %d\n", event_count[i]);
		total += event_count[i];
		if (!event_count[i])
			nohit++;
	} 

	printf("total events is: %d\n", total);
	printf("nohit is: %d\n", nohit);
}


Jason Baron (2):
  sched/wait: add round robin wakeup mode
  epoll: introduce EPOLLEXCLUSIVE and EPOLLROUNDROBIN

 fs/eventpoll.c                 | 25 ++++++++++++++++++++-----
 include/linux/wait.h           | 11 +++++++++++
 include/uapi/linux/eventpoll.h |  6 ++++++
 kernel/sched/wait.c            |  5 ++++-
 4 files changed, 41 insertions(+), 6 deletions(-)

-- 
1.8.2.rc2

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