[PATCH] x86/mm/cpa: avoid wbinvd() for PREEMPT_RT_FULL

[John Ogness <john.ogness@linutronix.de>]

Although wbinvd() is faster than flushing many individual pages, it
blocks the memory bus for "long" periods of time (>100us), thus
directly causing unusually large latencies for PREEMPT_RT_FULL. For
1024 pages, flushing those pages individually can take up to 2200us,
but the task remains fully preemptible during that time.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 arch/x86/mm/pageattr.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index e3353c9..a182477 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -214,7 +214,12 @@ static void cpa_flush_array(unsigned long *start, int numpages, int cache,
 			    int in_flags, struct page **pages)
 {
 	unsigned int i, level;
+#ifdef CONFIG_PREEMPT_RT_FULL
+	/* wbinvd() causes ugly latencies, avoid it */
+	unsigned long do_wbinvd = 0;
+#else
 	unsigned long do_wbinvd = cache && numpages >= 1024; /* 4M threshold */
+#endif
 
 	BUG_ON(irqs_disabled());
 
-- 
2.9.3