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

From: Peter Zijlstra
Date: Fri Sep 07 2018 - 08:26:04 EST


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);
}