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x86/mm/cpa: Avoid wbinvd() for PREEMPT 

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 on all CPUs, regardless of any CPU isolation
features that may be active. This is an unpriviledged operatation as it is
exposed to user space via the graphics subsystem.

For 1024 pages, flushing those pages individually can take up to 2200us,
but the task remains fully preemptible during that time.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: linux-rt-users <linux-rt-users@vger.kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
John Ogness 2017-01-30 09:41:21 +01:00 committed by Thomas Gleixner
parent f58576666c
commit 459fbe0069
1 changed files with 13 additions and 0 deletions
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13
arch/x86/mm/pageattr.c
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@ -214,7 +214,20 @@ 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
/*
* Avoid wbinvd() because it causes latencies on all CPUs,
* regardless of any CPU isolation that may be in effect.
*
* This should be extended for CAT enabled systems independent of
* PREEMPT because wbinvd() does not respect the CAT partitions and
* this is exposed to unpriviledged users through the graphics
* subsystem.
*/
unsigned long do_wbinvd = 0;
#else
unsigned long do_wbinvd = cache && numpages >= 1024; /* 4M threshold */
#endif
BUG_ON(irqs_disabled());