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SubjectRe: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly section
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On Tuesday, May 22, 2012 18:55:41 Vlad Zolotarov wrote:
> > > I have no fundamental prefer to either approach, but the
> > > direction taken should be justified explicitly, with numbers,
> > > arguments, etc. - also a short blurb somewhere in the headers
> > > that explains when they should be used, so that others can be
> > > aware of vSMP's special needs here.
> >
> > I.e. *numbers* are needed: roughly how many percpu variables in
> > a defconfig of one type versus the other type. This settles the
> > question whether we want to identify read-mostly or
> > write-frequently variables, to address this particular problem
> > ...
>

Ingo, here is the proposal to the patch (series) description:

------------------------------------------------------------------------------------------------
Added "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.
---------------------------------------------------------------------------------------------------

Pls., tell me if the above looks satisfactory to u in light of all your
previous remarks.

If yes - I'll respin the series with the description above.

thanks,
vlad


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