On Mon, Sep 17, 2018 at 02:05:40PM -0700, Subhra Mazumdar wrote:
> On 09/07/2018 05:25 AM, Peter Zijlstra wrote:
> >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);
> > }
> This will make the current thread always spin and block as it itself does
> the state change to TASK_RUNNING in finish_wait.
Nah, the actual wakeup will also do that state change. The one in
finish_wait() is for the case where the wait condition became true
without wakeup, such that we don't 'leak' the INTERRUPTIBLE state.
