Re: [RFC] per-cpu preempt_count

["H. Peter Anvin" <hpa@zytor.com>]

On 08/12/2013 12:00 PM, Linus Torvalds wrote:
> 
> Wrong. The thing is, the common case for preempt is to increment and
> decrement the count, not testing it. Exactly because we do this for
> spinlocks and for rcu read-locked regions.
> 
> Now, what we *could* do is to say:
> 
>  - we will use the high bit of the preempt count for NEED_RESCHED
> 
>  - when we set/clear that high bit, we *always* use atomic sequences,
> and we never change any of the other bits.
> 
>  - we will increment/decrement the other counters, we *only* do so on
> the local CPU, and we don't use atomic accesses.
> 
> Now, the downside of that is that *because* we don't use atomic
> accesses for the inc/dec parts, the updates to the high bit can get
> lost. But because the high bit updates are done with atomics, we know
> that they won't mess up the actual counting bits, so at least the
> count is never corrupted.
> 
> And the NEED_RESCHED bit getting lost would be very unusual. That
> clearly would *not* be acceptable for RT, but it it might be
> acceptable for "in the unusual case where we want to preempt a thread
> that was not preemtible, *and* we ended up having the extra unsual
> case that preemption enable ended up missing the preempt bit, we don't
> get preempted in a timely manner". It's probably impossible to ever
> see in practice, and considering that for non-RT use the PREEMPT bit
> is a "strong hint" rather than anything else, it sounds like it might
> be acceptable.
> 
> It is obviously *not* going to be acceptable for the RT people,
> though, but since they do different code sequences _anyway_, that's
> not really much of an issue.
> 

This seems more pain than need be if checking the count in the slow path
is okay.

	-hpa