Messages in this thread | | | From | Vlad Zolotarov <> | Subject | Re: [PATCH v4 0/2] Move x86_cpu_to_apicid to the __read_mostly section | Date | Wed, 23 May 2012 12:16:29 +0300 |
| |
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
| |