Re: [PATCH 3/4] sched: Pull preemption disablement to __schedule() caller

Peter Zijlstra
2015-02-03T10:53:25Z

On Mon, Feb 02, 2015 at 06:53:45PM +0100, Frederic Weisbecker wrote:
> It looks like preempt_count_add/inc() mostly imply entering a context that we want
> to be seen right away (thus want barrier() after) and preempt_count_sub/dec() mostly
> want previous work to be visible before re-enabling interrupt, preemption, etc...
> (thus want barrier() before).
>
> So maybe these functions (the non-underscored ones) should imply a barrier() rather
> than their callers (preempt_disable() and others). Inline functions instead of macros
> would do the trick (if the headers hell let us do that).
>
> Note the underscored implementations are all inline currently so this happens to
> work by chance for direct calls to preempt_count_add/sub() outside preempt_disable().
> If the non-underscored caller is turned into inline too I don't expect performance issues.
>
> What do you think, does it make sense?

AFAIK inline does _not_ guarantee a compiler barrier, only an actual
function call does.

When inlining the compiler creates visibility into the 'call' and can
avoid the constraint -- teh interweb seems to agree and also pointed out
that 'pure' function calls, even when actual function calls, can avoid
being a compiler barrier.

The below blog seems to do a fair job of explaining things; in
particular the 'implied compiler barriers' section is relevant here:

http://preshing.com/20120625/memory-ordering-at-compile-time/

As it stands the difference between the non underscore and the
underscore version is debug/tracing muck. The underscore ops are the raw
operations without fancy bits on.

I think I would prefer keeping it that way; this means that
preempt_count_$op() is a pure op and when we want to build stuff with it
like preempt_{en,dis}able() they add the extra semantics on top.

In any case; if we make __schedule() noinline (I think that might make
sense) that function call would itself imply the compiler barrier and
something like:

__preempt_count_add(PREEMPT_ACTIVE + PREEMPT_CHECK_OFFSET);
__schedule();
__preempt_count_sub(PREEMPT_ACTIVE + PREEMPT_CHECK_OFFSET);

Would actually be safe/correct.

As it stands I think __schedule() would fail the GCC inline static
criteria for being too large, but you never know, noinline guarantees it
will not.