From: Scott Moser <smoser@redhat.com>
Subject: [RHEL5.1 PATCH] bz242762 [PPC] donate cycles from dedicated cpu
Date: Wed, 6 Jun 2007 14:52:22 -0400 (EDT)
Bugzilla: 242762
Message-Id: <Pine.LNX.4.64.0706061448010.7066@squad5-lp1.lab.boston.redhat.com>
Changelog: [ppc64] donate cycles from dedicated cpu



RHBZ#: 242762 [FEATURE]
------
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=242762

Description:
------------
A Power6 can give up CPU cycles on a dedicated CPU (as opposed to a
shared CPU) to other shared processors if the administrator asks for it
(via the HMC).

This patch enables that to work properly on P6.

This just involves setting a bit in the CAS structure as well as the
VPA.  To donate cycles, a CPU has to have all SMT threads idle and w/
donate bit set in the VPA.  Then call H_CEDE.

Signed-off-by: Jake Moilanen <moilanen at austin.ibm.com>

RHEL Version Found:
-------------------
requested as a feature for 5.1

Upstream Status:
----------------
This patch has been submitted for upstream acceptance [1].

Test Status:
------------
A brew scratch build of 2.6.18-19.el5 with this patch applied
is available at [2]. 

This build has been booted on both power5 and power6 machines.

Jake Moillen of IBM has tested the patch when applied on top of the RHEL 5
GA kernel.

Proposed Patch:
----------------
Please review and ACK for RHEL5.1

-- 
[1] http://ozlabs.org/pipermail/linuxppc-dev/2007-June/thread.html#37098
[2] http://brewweb.devel.redhat.com/brew/taskinfo?taskID=808334 .

---
 kernel/prom_init.c        |    3 ++-
 kernel/sysfs.c            |    3 +--
 platforms/pseries/setup.c |   21 +++------------------
 3 files changed, 6 insertions(+), 21 deletions(-)
 
Index: b/arch/powerpc/kernel/prom_init.c
===================================================================
--- a/arch/powerpc/kernel/prom_init.c
+++ b/arch/powerpc/kernel/prom_init.c
@@ -635,6 +635,7 @@ static void __init early_cmdline_parse(v
 /* ibm,dynamic-reconfiguration-memory property supported */
 #define OV5_DRCONF_MEMORY	0x20
 #define OV5_LARGE_PAGES		0x10	/* large pages supported */
+#define OV5_DONATE_DEDICATE_CPU 0x02   /* donate dedicated CPU support */
 
 /*
  * The architecture vector has an array of PVR mask/value pairs,
@@ -679,7 +680,7 @@ static unsigned char ibm_architecture_ve
 	/* option vector 5: PAPR/OF options */
 	3 - 2,				/* length */
 	0,				/* don't ignore, don't halt */
-	OV5_LPAR | OV5_SPLPAR | OV5_LARGE_PAGES,
+	OV5_LPAR | OV5_SPLPAR | OV5_LARGE_PAGES | OV5_DONATE_DEDICATE_CPU,
 };
 
 /* Old method - ELF header with PT_NOTE sections */
Index: b/arch/powerpc/platforms/pseries/setup.c
===================================================================
--- a/arch/powerpc/platforms/pseries/setup.c
+++ b/arch/powerpc/platforms/pseries/setup.c
@@ -470,6 +470,7 @@ static void pseries_dedicated_idle_sleep
 	 * a good time to find other work to dispatch.
 	 */
 	get_lppaca()->idle = 1;
+	get_lppaca()->cpuctls_task_attrs = 1;
 
 	/*
 	 * We come in with interrupts disabled, and need_resched()
@@ -498,27 +499,11 @@ static void pseries_dedicated_idle_sleep
 			goto out;
 	}
 
-	/*
-	 * If not SMT, cede processor.  If CPU is running SMT
-	 * cede if the other thread is not idle, so that it can
-	 * go single-threaded.  If the other thread is idle,
-	 * we ask the hypervisor if it has pending work it
-	 * wants to do and cede if it does.  Otherwise we keep
-	 * polling in order to reduce interrupt latency.
-	 *
-	 * Doing the cede when the other thread is active will
-	 * result in this thread going dormant, meaning the other
-	 * thread gets to run in single-threaded (ST) mode, which
-	 * is slightly faster than SMT mode with this thread at
-	 * very low priority.  The cede enables interrupts, which
-	 * doesn't matter here.
-	 */
-	if (!cpu_has_feature(CPU_FTR_SMT) || !lppaca[cpu ^ 1].idle
-	    || poll_pending() == H_PENDING)
-		cede_processor();
+	cede_processor();
 
 out:
 	HMT_medium();
+	get_lppaca()->cpuctls_task_attrs = 0;
 	get_lppaca()->idle = 0;
 }
 
Index: b/arch/powerpc/kernel/sysfs.c
===================================================================
--- a/arch/powerpc/kernel/sysfs.c
+++ b/arch/powerpc/kernel/sysfs.c
@@ -25,8 +25,7 @@ static DEFINE_PER_CPU(struct cpu, cpu_de
 /* SMT stuff */
 
 #ifdef CONFIG_PPC_MULTIPLATFORM
-/* default to snooze disabled */
-DEFINE_PER_CPU(unsigned long, smt_snooze_delay);
+DEFINE_PER_CPU(unsigned long, smt_snooze_delay) = { 100 };
 
 static ssize_t store_smt_snooze_delay(struct sys_device *dev, const char *buf,
 				      size_t count)