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Re: [Xen-devel] [PATCH] x86: Use deep C states for off-lined CPUs

As far as I can tell the most relevant change in Linux was this: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=ea53069231f9317062910d6e772cca4ce93de8c8 and it sounds that it was made mostly because MWAIT-based idle is more efficient on Intel processors. That's not the case on AMD where IO-based idle is preferred (and I am not aware of any issues, at least so far).

I can make the patch to be AMD_specific but since for the most parts the logic is the same as in acpi_idle_do_entry() won't we have to modify that function as well?


On 02/28/12 23:58, Liu, Jinsong wrote:
I don't think we should go back to old SYSIO method, the history here is:

Xen originally has SYSIO method when offline cpu, but at c/s 23022 we cancel it 
as reason below
x86: Fix cpu offline bug: cancel SYSIO method when play dead

Play dead is a fragile and tricky point of cpu offline logic.  For how
to play cpu dead, linux kernel changed several times: Very old kernel
support 3 ways to play cpu dead: mwait, SYSIO, and halt, just like
what cpuidle did when enter C3; Later, it cancel mwait and SYSIO
support, only use halt to play dead; Latest linux 2.6.38 add mwait
support when cpu dead.

This patch cancel SYSIO method when cpu dead, keep same with latest

SYSIO is an obsoleted method to enter deep C, with some tricky
hardware behavior, and seldom supported in new platform.  Xen
experiment indicate that when cpu dead, SYSIO method would trigger
unknown issue which would bring strange error.  We now cancel SYSIO
method when cpu dead, after all, correctness is more important than
power save, and btw new platform use mwait.


Boris Ostrovsky wrote:
# HG changeset patch
# User Boris Ostrovsky<boris.ostrovsky@xxxxxxx>
# Date 1330466573 -3600
# Node ID 9e5991ad9c85b5176ce269001e7957e8805dd93c
# Parent  a7bacdc5449a2f7bb9c35b2a1334b463fe9f29a9
x86: Use deep C states for off-lined CPUs

Currently when a core is taken off-line it is placed in C1 state
(unless MONITOR/MWAIT is used). This patch allows a core to go to
deeper C states resulting in significantly higher power savings.

Signed-off-by: Boris Ostrovsky<boris.ostrovsky@xxxxxxx>

diff -r a7bacdc5449a -r 9e5991ad9c85 xen/arch/x86/acpi/cpu_idle.c
--- a/xen/arch/x86/acpi/cpu_idle.c      Mon Feb 27 17:05:18 2012 +0000
+++ b/xen/arch/x86/acpi/cpu_idle.c      Tue Feb 28 23:02:53 2012 +0100
@@ -573,10 +573,10 @@ static void acpi_dead_idle(void)
      if ( (cx =&power->states[power->count-1]) == NULL )
          goto default_halt;

-    mwait_ptr = (void *)&mwait_wakeup(smp_processor_id());
      if ( cx->entry_method == ACPI_CSTATE_EM_FFH )
+        mwait_ptr = (void *)&mwait_wakeup(smp_processor_id());
           * Cache must be flushed as the last operation before
           * Otherwise, CPU may still hold dirty data, breaking cache
coherency, @@ -601,6 +601,20 @@ static void acpi_dead_idle(void)
              __mwait(cx->address, 0);
+    }
+    else if ( cx->entry_method == ACPI_CSTATE_EM_SYSIO )
+    {
+        /* Avoid references to shared data after the cache flush */
+        u32 address = cx->address;
+        u32 pmtmr_ioport_local = pmtmr_ioport;
+        wbinvd();
+        while ( 1 )
+        {
+            inb(address);
+            inl(pmtmr_ioport_local);
+        }


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