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);
 }