Messages in this thread | | | Date | Tue, 3 Feb 2015 11:53:03 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 3/4] sched: Pull preemption disablement to __schedule() caller |
| |
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.
| |