linux-kernel - Re: [GIT PULL] scheduler fixes

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Date:	Mon, 18 May 2009 21:03:20 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jeff Garzik <jgarzik@...ox.com>,
	Alexander Viro <viro@....linux.org.uk>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [GIT PULL] scheduler fixes


* Ingo Molnar <mingo@...e.hu> wrote:

> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > On Mon, 18 May 2009, Ingo Molnar wrote:
> > > 
> > > hm - i've Cc:-ed Jeff & Viro. The background is that Sparse and LLVM 
> > > barfed on the current construct and Al strongly advocated this 
> > > solution, see:
> > 
> > I know the background.
> > 
> > Did you read my email?
> > 
> > Did you see my one-line patch that fixes the same problem WITHOUT 
> > THE INSANITY?
> 
> Yes, i even proposed that very patch in the original discussion, in 
> my first reply:
> 
>     http://lkml.org/lkml/2009/5/8/378

Something like the patch below. It also fixes ->span[] which has a 
similar problem.

But ... i think this needs further clean-ups really. Either go fully 
static, or go fully dynamic.

I'd suggest we go fully dynamic: the static structures are never 
used directly anyway, they are pointer-ized during sched domain 
setup.

The reason for this duality is allocation bootstrap: there's no 
generic early-capable allocator that allocates something and 
switches from bootmem to kmalloc transparently once the SLAB has 
been set up.

Would be nice if bootmem_alloc() was extended with such properties - 
if SLAB is up (and bootmem is down) it would return 
kmalloc(GFP_KERNEL) memory buffers.

	Ingo

-------------->
Subject: sched: properly define the sched_group::cpumask and sched_domain::span fields
From: Ingo Molnar <mingo@...e.hu>

Properly document the variable-size structure tricks we are doing
wrt. struct sched_group and sched_domain, and use the field[0] GCC
extension instead of defining a vla array.

Dont use unions for this, as pointed out by Linus.

This also un-confuses Sparse and LLVM.

Reported-by: Jeff Garzik <jeff@...zik.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
LKML-Reference: <alpine.LFD.2.01.0905180850110.3301@...alhost.localdomain>
Not-Yet-Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 include/linux/sched.h |   25 ++++++++++++++++++++++---
 kernel/sched.c        |    5 +++--
 2 files changed, 25 insertions(+), 5 deletions(-)

Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -846,7 +846,17 @@ struct sched_group {
 	 */
 	u32 reciprocal_cpu_power;
 
-	unsigned long cpumask[];
+	/*
+	 * The CPUs this group covers.
+	 *
+	 * NOTE: this field is variable length. (Allocated dynamically
+	 * by attaching extra space to the end of the structure,
+	 * depending on how many CPUs the kernel has booted up with)
+	 *
+	 * It is also be embedded into static data structures at build
+	 * time. (See 'struct static_sched_group' in kernel/sched.c)
+	 */
+	unsigned long cpumask[0];
 };
 
 static inline struct cpumask *sched_group_cpus(struct sched_group *sg)
@@ -932,8 +942,17 @@ struct sched_domain {
 	char *name;
 #endif
 
-	/* span of all CPUs in this domain */
-	unsigned long span[];
+	/*
+	 * Span of all CPUs in this domain.
+	 *
+	 * NOTE: this field is variable length. (Allocated dynamically
+	 * by attaching extra space to the end of the structure,
+	 * depending on how many CPUs the kernel has booted up with)
+	 *
+	 * It is also be embedded into static data structures at build
+	 * time. (See 'struct static_sched_domain' in kernel/sched.c)
+	 */
+	unsigned long span[0];
 };
 
 static inline struct cpumask *sched_domain_span(struct sched_domain *sd)
Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -8049,8 +8049,9 @@ int sched_smt_power_savings = 0, sched_m
 
 /*
  * The cpus mask in sched_group and sched_domain hangs off the end.
- * FIXME: use cpumask_var_t or dynamic percpu alloc to avoid wasting space
- * for nr_cpu_ids < CONFIG_NR_CPUS.
+ *
+ * ( See the the comments in include/linux/sched.h:struct sched_group
+ *   and struct sched_domain. )
  */
 struct static_sched_group {
 	struct sched_group sg;
--
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