linux-kernel - [PATCH v5 0/2] Move x86_cpu_to_apicid to the __read

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Date:	Mon, 11 Jun 2012 12:56:41 +0300
From:	Vlad Zolotarov <vlad@...lemp.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ido Yariv <ido@...ery.com>,
	"Shai Fultheim (Shai@...leMP.com)" <Shai@...lemp.com>
Subject: [PATCH v5 0/2] Move x86_cpu_to_apicid to the __read_mostly section

Pls., consider applying this patch series.
It contains the following changes:
 - Adds two new macros DEFINE_EARLY_PER_CPU_READ_MOSTLY() and
   DECLARE_EARLY_PER_CPU_READ_MOSTLY().
 - Adds "read-mostly" qualifier to the following variables in smp.h:
  - cpu_sibling_map
  - cpu_core_map
  - cpu_llc_shared_map
  - cpu_llc_id
  - cpu_number
  - x86_cpu_to_apicid
  - x86_bios_cpu_apicid
  - x86_cpu_to_logical_apicid

As long as all the variables above are only written during the initialization,
this change is meant to prevent the false sharing. More specifically, on vSMP 
Foundation platform x86_cpu_to_apicid shared the same internode_cache_line 
with frequently written lapic_events.

>From the analysis of the first 33 per_cpu variables  out of 219 (memories they 
describe, to be more specific) the 8 have read_mostly nature 
(tlb_vector_offset, cpu_loops_per_jiffy, xen_debug_irq, etc.) and 25 are 
frequently written (irq_stack_union, gdt_page, exception_stacks, idt_desc, 
etc.). Assuming that the spread of the rest of the per_cpu variables is 
similar, identifying the read mostly memories will make more sense in terms of 
long-term code maintenance comparing to identifying frequently written 
memories.

v5 changes:
- Changed the description of the patch series and patch 2:
  added the analysis of the current per-cpu varibles spread
  between the read-mostly and frequently written groups.

v4 changes:
- Fixed the authors signatures in the patches.

v3 changes:
- Added the missing definitions of DEFINE_EARLY_PER_CPU_READ_MOSTLY()
  and DECLARE_EARLY_PER_CPU_READ_MOSTLY() macros in the !CONFIG_SMP code
  path in arch/x86/include/asm/percpu.h.

thanks,
vlad

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