Messages in this thread | | | Date | Mon, 21 May 2012 22:19:58 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly section |
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* Vlad Zolotarov <vlad@scalemp.com> wrote:
> On Monday, May 21, 2012 17:23:48 Ingo Molnar wrote: > > * Vlad Zolotarov <vlad@scalemp.com> wrote: > > > 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 and improve the performance on large multiprocessor > > > systems. > > > > Why have you resent this? The feedback I gave has not been > > > > addressed: > > Hmmm... I'm a bit confused. There were two feedbacks/threads: > one on "Signed- off-by" format and the other where u asked for > a justification on a vSMP side. > > The signed-off format sounded to me as a clear blocker for a > series so I fixed it and respined. I also mentioned it in > patch0.
Well, you need to address all blockers before we can proceed.
> The second thread seams like getting to submitting a separate > patch with a doc under Documents and vSMP testing results > explaining and justifying when and were per-CPU and/or > __read_mostly variables should be used.
No. As I said I'm not convinced that there are fewer read-mostly than read-write percpu variables. Please:
> Well, a quick tally of percpu variables on a 'make defconfig' > kernel would tell us one way or another? > > Here there's almost 200 percpu variables active in the 64-bit > x86 defconfig, and a quick random sample suggests that most > are read-mostly. > > 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 ...
Thanks,
Ingo
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