Re: [RFC PATCH 2/2] pipe: use pipe busy wait

[Peter Zijlstra]

On Thu, Aug 30, 2018 at 01:24:58PM -0700, subhra mazumdar wrote:
> +void pipe_busy_wait(struct pipe_inode_info *pipe)
> +{
> +     unsigned long wait_flag = pipe->pipe_wait_flag;
> +     unsigned long start_time = pipe_busy_loop_current_time();
> +
> +     pipe_unlock(pipe);
> +     preempt_disable();
> +     for (;;) {
> +             if (pipe->pipe_wait_flag > wait_flag) {
> +                     preempt_enable();
> +                     pipe_lock(pipe);
> +                     return;
> +             }
> +             if (pipe_busy_loop_timeout(pipe, start_time))
> +                     break;
> +             cpu_relax();
> +     }
> +     preempt_enable();
> +     pipe_lock(pipe);
> +     if (pipe->pipe_wait_flag > wait_flag)
> +             return;
> +     pipe_wait(pipe);
> +}
> +
> +void wake_up_busy_poll(struct pipe_inode_info *pipe)
> +{
> +     pipe->pipe_wait_flag++;
> +}

Why not just busy wait on current->state ? A little something like:

diff --git a/fs/pipe.c b/fs/pipe.c
index bdc5d3c0977d..8d9f1c95ff99 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -106,6 +106,7 @@ void pipe_double_lock(struct pipe_inode_info *pipe1,
 void pipe_wait(struct pipe_inode_info *pipe)
 {
        DEFINE_WAIT(wait);
+       u64 start;
 
        /*
         * Pipes are system-local resources, so sleeping on them
@@ -113,7 +114,15 @@ void pipe_wait(struct pipe_inode_info *pipe)
         */
        prepare_to_wait(&pipe->wait, &wait, TASK_INTERRUPTIBLE);
        pipe_unlock(pipe);
-       schedule();
+
+       preempt_disable();
+       start = local_clock();
+       while (!need_resched() && current->state != TASK_RUNNING &&
+                       (local_clock() - start) < pipe->poll_usec)
+               cpu_relax();
+       schedule_preempt_disabled();
+       preempt_enable();
+
        finish_wait(&pipe->wait, &wait);
        pipe_lock(pipe);
 }