一个nginx 回源限速的bug处理过程记录

一个生产环境，nginx占用cpu很高。

top - :: up  day, :,   users,  load average: 13.26, 13.20, 13.20
Tasks:  total,   running,  sleeping,    stopped,    zombie
Cpu(s): 21.3%us, 18.0%sy,  0.0%ni, 57.4%id,  0.0%wa,  0.0%hi,  3.3%si,  0.0%st
Mem:    257556M total,   254371M used,     3184M free,      400M buffers
Swap:        0M total,        0M used,        0M free,   200639M cached

  PID USER      PR  NI  VIRT  RES  SHR S   %CPU %MEM    TIME+  COMMAND
 root          .9g .3g .2g R      1.3 :16.31 nginx
   root          .9g .5g .4g R       1.4 :16.87 nginx
   root          .9g .5g .4g R       1.4 :35.20 nginx
   root          .9g .6g .5g R       1.4 :12.50 nginx
   root          .9g .5g .4g R       1.4 :04.89 nginx
 root          .9g .7g .6g R       1.5 :46.03 nginx
 root          .9g .6g .5g R       1.4 :45.09 nginx
 root          .9g .7g .6g R       1.5 :42.75 nginx

top跟进去单个线程，发现是主线程很高：

top - :: up  day, :,   users,  load average: 12.79, 12.84, 13.04
Tasks:   total,    running,   sleeping,    stopped,    zombie
Cpu(s): 18.0%us, 21.5%sy,  0.1%ni, 54.6%id,  1.7%wa,  0.0%hi,  4.1%si,  0.0%st
Mem:    257556M total,   253504M used,     4052M free,      400M buffers
Swap:        0M total,        0M used,        0M free,   199627M cached

  PID USER      PR  NI  VIRT  RES  SHR S   %CPU %MEM    TIME+  COMMAND
 root          .9g .4g .3g S        1.3 :24.72 nginx
  root          .9g .4g .3g S        1.3   :07.05 nginx
  root          .9g .4g .3g S        1.3   :06.36 nginx
  root          .9g .4g .3g S        1.3   :07.07 nginx
  root          .9g .4g .3g S        1.3   :06.18 nginx
  root          .9g .4g .3g S        1.3   :06.70 nginx
  root          .9g .4g .3g S        1.3   :03.87 nginx
  root          .9g .4g .3g S        1.3   :06.16 nginx
  root          .9g .4g .3g S        1.3   :06.51 nginx
  root          .9g .4g .3g S        1.3   :07.03 nginx
  root          .9g .4g .3g S        1.3   :07.10 nginx

gdb跟踪一下，发现大量的epoll_wait返回，中间没有任何系统调用：

epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) =
epoll_wait(, {{EPOLLOUT, {u32=, u64=}}}, , ) = 

查看堆栈，

epoll_wait在返回有active的fd的时候，我们并没有去调用recv或者recvfrom，走查代码：

        if ((revents & EPOLLOUT) && wev->active) {

            if (c->fd == - || wev->instance != instance) {

                /*
                 * the stale event from a file descriptor
                 * that was just closed in this iteration
                 */

                ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, ,
                               "epoll: stale event %p", c);
                continue;
            }

            wev->ready = ;

            if (flags & NGX_POST_EVENTS) {
                ngx_post_event(wev, &ngx_posted_events);

            } else {
                wev->handler(wev);
            }
        }

发现 wev->active 有时候不为1，但是在epoll_wait返回的时候，大多数情况是1，为什么没有recv呢？

查看 wev->handler，在 ngx_http_upstream_process_non_buffered_request 中，有这么一个分支判断：

            if(downstream->write->delayed)
            {
                ngx_log_error(NGX_LOG_DEBUG, upstream->log, ,
                    "[no_buffering_limite_rate] downstream->write->delayed return here. remove upstream read event");

                ngx_del_event(upstream->read, NGX_READ_EVENT, );
                return;
            }

在做了限速的情况下，只删除了 upstream->read 事件，没有删除 upstream->write 事件。

而我们epoll_wait返回的是 EPOLLOUT 事件。

在这种情况下，wev->handler(wev); 由于处理非常快，

ngx_process_events_and_timers(ngx_cycle_t *cycle)函数在下面这个逻辑：

    delta = ngx_current_msec;

    (void) ngx_process_events(cycle, timer, flags);

    delta = ngx_current_msec - delta;

这样delta几乎为0，使得下次循环的时候扫描的timer为红黑树的最小值，且这个值都没有超时。

 timer = ngx_event_find_timer();

所以我们也看到epoll_wai的超时值大多数时间非常小。

最后，这个问题在满足限速情况下，将NGX_WRITE_EVENT 从epoll中删除，等到了限速不满足的时候，再加入这个event。