lkml.org 
[lkml]   [2010]   [Sep]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [tip:x86/idle] x86, hotplug: Move WBINVD back outside the play_dead loop
On 09/20/2010 11:20 AM, H. Peter Anvin wrote:
> On 09/17/2010 05:48 PM, Venkatesh Pallipadi wrote:
>>>
>>> + wbinvd();
>>> +
>>> while (1) {
>>> - mb();
>>> - wbinvd();
>>> __monitor(&current_thread_info()->flags, 0, 0);
>>> mb();
>>
>>
>> Just one observation. There are some CPUs with errata that need
>> clflush before monitor. So, if that CPU wakesup spuriously it may have
>> problem reentering idle. Not sure whether that will be a problem as
>> that errata also depended on read happening on the flag. May be its
>> better to do monitor (0, 0, 0).
>>
>
> It seems the easy way to deal with that would be to just add clflush
> before monitor... it is *probably* redundant, but it should be safe to
> do. It means depending on X86_FEATURE_CLFLUSH as well as
> X86_FEATURE_MWAIT, but I don't think there is any x86 processor which
> has MWAIT and not CLFLUSH, and I highly doubt there ever will be.
>
> Does anyone see any downside?
>

Patch for review... anyone see any problem with it?

-hpa

From 83c70ace9aad19ba7c41986f3c34a0808fbaf78f Mon Sep 17 00:00:00 2001
From: H. Peter Anvin <hpa@linux.intel.com>
Date: Mon, 20 Sep 2010 13:04:45 -0700
Subject: [PATCH] x86, hotplug: In the MWAIT case of play_dead, CLFLUSH the cache line

When we're using MWAIT for play_dead, explicitly CLFLUSH the cache
line before executing MONITOR. This is a potential workaround for the
Xeon 7400 erratum AAI65 after having a spurious wakeup and returning
around the loop. "Potential" here because it is not certain that that
erratum could actually trigger; however, the CLFLUSH should be
harmless.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan van de Ven <arjan@linux.kernel.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Venkatesh Pallipadi <venki@google.com>
---
arch/x86/kernel/smpboot.c | 20 +++++++++++++++++++-
1 files changed, 19 insertions(+), 1 deletions(-)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 55c80ffb..fdccfe9 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1394,9 +1394,12 @@ static inline void mwait_play_dead(void)
unsigned int highest_cstate = 0;
unsigned int highest_subcstate = 0;
int i;
+ void *mwait_ptr;

if (!cpu_has(&current_cpu_data, X86_FEATURE_MWAIT))
return;
+ if (!cpu_has(&current_cpu_data, X86_FEATURE_CLFLSH))
+ return;
if (current_cpu_data.cpuid_level < CPUID_MWAIT_LEAF)
return;

@@ -1422,10 +1425,25 @@ static inline void mwait_play_dead(void)
(highest_subcstate - 1);
}

+ /*
+ * This should be a memory location in a cache line which is
+ * unlikely to be touched by other processors. The actual
+ * content is immaterial as it is not actually modified in any way.
+ */
+ mwait_ptr = &current_thread_info()->flags;
+
wbinvd();

while (1) {
- __monitor(&current_thread_info()->flags, 0, 0);
+ /*
+ * The CLFLUSH is a workaround for erratum AAI65 for
+ * the Xeon 7400 series. It's not clear it is actually
+ * needed, but it should be harmless in either case.
+ * The WBINVD is insufficient due to the spurious-wakeup
+ * case where we return around the loop.
+ */
+ clflush(mwait_ptr);
+ __monitor(mwait_ptr, 0, 0);
mb();
__mwait(eax, 0);
}
--
1.7.2
\
 
 \ /
  Last update: 2010-09-20 22:15    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site