Re: [RFC PATCH 03/10] ftrace: Drop the ftrace_profile_enabled checks in tracing hot path

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Jan 21, 2010 at 10:05:06PM -0500, Steven Rostedt wrote:
> On Fri, 2010-01-22 at 03:43 +0100, Frederic Weisbecker wrote:
> 
> > > Now for the reason I Cc'd Paul and Mathieu...
> > > 
> > > If we had a synchronize_sched() like function that would wait and return
> > > when all preempted tasks have been scheduled again and went to either
> > > userspace or called schedule directly, then we could actually do this.
> > > 
> > > After unregistering the function graph trace, you call this
> > > "synchronize_tasks()" and it will guarantee that all currently preempted
> > > tasks have either went to userspace or have called schedule() directly.
> > > Then it would be safe to remove this check.
> > 
> > 
> > 
> > Good point!
> > 
> > I fear that would require heavy hooks in the scheduler though...
> > 
> 
> Not a heavy one. We could add a field to the task_struct and just call
> something if it is set.
> 
> 
> At start of schedule()
> 
> 	if (unlikely(current->pcount))
> 		handle_pcount_waiters(current);
> 
> 
> void handle_pcount_waiters(struct task_struct *p)
> {
> 	current->pcount = 0;
> 	wake_up(pcount_waiters);
> }
> 
> 
> and for the synchronize_tasks(), just search the task list for tasks
> that are on the run queue but not running, and add a pcount timestamp
> and record the list of tasks (allocated list).
> 
> After it is woken up, it checks the list of tasks and if a task does not
> have the pcount timestamp that matches what was stored, it removes it
> from the list. When it is finally woken up and does not have any more
> tasks on the list, it continues.
> 
> This is just a basic idea, i left out a bunch of details, but I'm sure
> it is feasible. This type of wait may work for other types of lockless
> algorithms too.
> 
> -- Steve



Sounds like a good idea. That would avoid these wasteful checks and
resident buffers.