Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly section

[Vlad Zolotarov <vlad@scalemp.com>]

On Monday 11 June 2012 11:00:31 Ingo Molnar wrote:
> * Vlad Zolotarov <vlad@scalemp.com> wrote:
> > On Wednesday 23 May 2012 12:16:29 Vlad Zolotarov wrote:
> > > 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.
> > 
> > Ingo, sorry for bothering. Could u, pls., tell if the above
> > description is ok? We'd like to move on with this patch
> > series.
> 
> Yeah, that description and analysis looks good and sensible.
> 
> Mind resending the updated patches in a new thread?

Sure. I'm on it right now, sir. :)

thanks,
vlad

> 
> Thanks,
> 
> 	Ingo