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Date: Wed, 20 Feb 2008 07:54:14 -0800
From: Mike Travis <travis@....com>
To: Eric Dumazet <dada1@...mosbay.com>
CC: Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>, Andi Kleen <ak@...e.de>,
Christoph Lameter <clameter@....com>,
Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Andy Whitcroft <apw@...dowen.org>,
Randy Dunlap <rdunlap@...otime.net>,
Joel Schopp <jschopp@...tin.ibm.com>
Subject: Re: [PATCH 1/2] x86_64: Fold pda into per cpu area v3
Eric Dumazet wrote:
> On Wed, 20 Feb 2008 13:07:47 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
>
>> * Mike Travis <travis@....com> wrote:
>>
>>> * Declare the pda as a per cpu variable. This will move the pda area
>>> to an address accessible by the x86_64 per cpu macros.
>>> Subtraction of __per_cpu_start will make the offset based from the
>>> beginning of the per cpu area. Since %gs is pointing to the pda,
>>> it will then also point to the per cpu variables and can be
>>> accessed thusly:
>>>
>>> %gs:[&per_cpu_xxxx - __per_cpu_start]
>> randconfig QA on x86.git found a crash on x86.git#testing with
>> nmi_watchdog=2 (config attached) - and i bisected it down to this patch.
>>
>> config and crashlog attached. You can pick up x86.git#testing via:
>>
>> http://people.redhat.com/mingo/x86.git/README
>>
>> (since i had to hand-merge the patch when integrating it, i've attached
>> the merged version below.)
>>
>> Ingo
>>
>> -------------->
>> Subject: x86_64: Fold pda into per cpu area v3
>> From: Mike Travis <travis@....com>
>> Date: Tue, 19 Feb 2008 12:33:36 -0800
>>
>> * Declare the pda as a per cpu variable. This will move the pda area
>> to an address accessible by the x86_64 per cpu macros. Subtraction
>> of __per_cpu_start will make the offset based from the beginning
>> of the per cpu area. Since %gs is pointing to the pda, it will
>> then also point to the per cpu variables and can be accessed thusly:
>>
>> %gs:[&per_cpu_xxxx - __per_cpu_start]
>>
>> * The boot_pdas are only needed in head64.c so move the declaration
>> over there. And since the boot_cpu_pda is only used during
>> bootup and then copied to the per_cpu areas during init, it is
>> then removable. In addition, the initial cpu_pda pointer table
>> is reallocated to be the correct size for the number of cpus.
>>
>> * Remove the code that allocates special pda data structures.
>> Since the percpu area is currently maintained for all possible
>> cpus then the pda regions will stay intact in case cpus are
>> hotplugged off and then back on.
>>
>> * Relocate the x86_64 percpu variables to begin at zero. Then
>> we can directly use the x86_32 percpu operations. x86_32
>> offsets %fs by __per_cpu_start. x86_64 has %gs pointing
>> directly to the pda and the per cpu area thereby allowing
>> access to the pda with the x86_64 pda operations and access
>> to the per cpu variables using x86_32 percpu operations.
>>
>> * This also supports further integration of x86_32/64.
>>
>> Cc: Andy Whitcroft <apw@...dowen.org>
>> Cc: Randy Dunlap <rdunlap@...otime.net>
>> Cc: Joel Schopp <jschopp@...tin.ibm.com>
>> Signed-off-by: Christoph Lameter <clameter@....com>
>> Signed-off-by: Mike Travis <travis@....com>
>> Signed-off-by: Ingo Molnar <mingo@...e.hu>
>> ---
>> arch/x86/Kconfig | 3 +
>> arch/x86/kernel/head64.c | 41 ++++++++++++++++++++++++
>> arch/x86/kernel/setup64.c | 66 ++++++++++++++++++++++++---------------
>> arch/x86/kernel/smpboot_64.c | 16 ---------
>> arch/x86/kernel/vmlinux_64.lds.S | 1
>> include/asm-x86/pda.h | 13 +++++--
>> include/asm-x86/percpu.h | 33 +++++++++++--------
>> 7 files changed, 115 insertions(+), 58 deletions(-)
>>
>> Index: linux-x86.q/arch/x86/Kconfig
>> ===================================================================
>> --- linux-x86.q.orig/arch/x86/Kconfig
>> +++ linux-x86.q/arch/x86/Kconfig
>> @@ -122,6 +122,9 @@ config ARCH_HAS_CPU_RELAX
>> config HAVE_SETUP_PER_CPU_AREA
>> def_bool X86_64
>>
>> +config HAVE_ZERO_BASED_PER_CPU
>> + def_bool X86_64
>> +
>> config ARCH_HIBERNATION_POSSIBLE
>> def_bool y
>> depends on !SMP || !X86_VOYAGER
>> Index: linux-x86.q/arch/x86/kernel/head64.c
>> ===================================================================
>> --- linux-x86.q.orig/arch/x86/kernel/head64.c
>> +++ linux-x86.q/arch/x86/kernel/head64.c
>> @@ -11,6 +11,7 @@
>> #include <linux/string.h>
>> #include <linux/percpu.h>
>> #include <linux/start_kernel.h>
>> +#include <linux/bootmem.h>
>>
>> #include <asm/processor.h>
>> #include <asm/proto.h>
>> @@ -23,6 +24,12 @@
>> #include <asm/kdebug.h>
>> #include <asm/e820.h>
>>
>> +#ifdef CONFIG_SMP
>> +/* Only used before the per cpu areas are setup. */
>> +static struct x8664_pda boot_cpu_pda[NR_CPUS] __initdata;
>> +static struct x8664_pda *_cpu_pda_init[NR_CPUS] __initdata;
>> +#endif
>> +
>> static void __init zap_identity_mappings(void)
>> {
>> pgd_t *pgd = pgd_offset_k(0UL);
>> @@ -102,8 +109,14 @@ void __init x86_64_start_kernel(char * r
>>
>> early_printk("Kernel alive\n");
>>
>> +#ifdef CONFIG_SMP
>> + _cpu_pda = (void *)_cpu_pda_init;
>> for (i = 0; i < NR_CPUS; i++)
>> cpu_pda(i) = &boot_cpu_pda[i];
>> +#endif
>> +
>> + /* setup percpu segment offset for cpu 0 */
>> + cpu_pda(0)->data_offset = (unsigned long)__per_cpu_load;
>>
>> pda_init(0);
>> copy_bootdata(__va(real_mode_data));
>> @@ -128,3 +141,31 @@ void __init x86_64_start_kernel(char * r
>>
>> start_kernel();
>> }
>> +
>> +#ifdef CONFIG_SMP
>> +/*
>> + * Remove initial boot_cpu_pda array and cpu_pda pointer table.
>> + *
>> + * This depends on setup_per_cpu_areas relocating the pda to the beginning
>> + * of the per_cpu area so that (_cpu_pda[i] != &boot_cpu_pda[i]). If it
>> + * is equal then the new pda has not been setup for this cpu, and the pda
>> + * table will have a NULL address for this cpu.
>> + */
>> +void __init x86_64_cleanup_pda(void)
>> +{
>> + int i;
>> +
>> + _cpu_pda = alloc_bootmem_low(nr_cpu_ids * sizeof(void *));
>
> Here we allocate an array of [nr_cpu_ids] slots
>
>> +
>> + if (!_cpu_pda)
>> + panic("Cannot allocate cpu pda table\n");
>> +
>> + /* cpu_pda() now points to allocated cpu_pda_table */
>> +
>> + for (i = 0; i < NR_CPUS; i++)
>
> But in this loop we want to read/write on [NR_CPUS] slots of this array
>
>> + if (_cpu_pda_init[i] == &boot_cpu_pda[i])
>> + cpu_pda(i) = NULL;
>> + else
>> + cpu_pda(i) = _cpu_pda_init[i];
>> +}
>> +#endif
>
> You might want to apply this patch.
>
> I also wonder if _cpu_pda should be set only at the very end of
> x86_64_cleanup_pda(), after array initialization, or maybe other
> cpus are not yet running ? (Sorry I cannot boot test this patch at this momeent)
>
> [PATCH] x86_64: x86_64_cleanup_pda() should use nr_cpu_ids instead of NR_CPUS
>
> We allocate an array of nr_cpu_ids pointers, so we should respect its bonds.
>
> Delay change of _cpu_pda after array initialization.
>
> Also take into account that alloc_bootmem_low() :
> - calls panic() if not enough memory
> - already clears allocated memory
>
> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
>
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 3942e6a..21532eb 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -154,18 +154,16 @@ void __init x86_64_start_kernel(char * real_mode_data)
> void __init x86_64_cleanup_pda(void)
> {
> int i;
> + struct x8664_pda **new_cpu_pda;
>
> - _cpu_pda = alloc_bootmem_low(nr_cpu_ids * sizeof(void *));
> + new_cpu_pda = alloc_bootmem_low(nr_cpu_ids * sizeof(void *));
>
> - if (!_cpu_pda)
> - panic("Cannot allocate cpu pda table\n");
>
> + for (i = 0; i < nr_cpu_ids; i++)
> + if (_cpu_pda_init[i] != &boot_cpu_pda[i])
> + new_cpu_pda[i] = _cpu_pda_init[i];
> + mb();
> + _cpu_pda = new_cpu_pda;
> /* cpu_pda() now points to allocated cpu_pda_table */
> -
> - for (i = 0; i < NR_CPUS; i++)
> - if (_cpu_pda_init[i] == &boot_cpu_pda[i])
> - cpu_pda(i) = NULL;
> - else
> - cpu_pda(i) = _cpu_pda_init[i];
> }
> #endif
Yes, thank you! I had changed this in a later version ... I must have grabbed
an earlier version to do the patch split. (But I did not have the memory barrier.)
At this point only the boot cpu is running and we're in the time slot between
allocating per cpu areas and sched_init().
I'm just building an x86.git#testing branch. I will see if I can't replicate
the problem.
Thanks again,
Mike
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