On Fri, Sep 09, 2005 at 10:07:28AM -0700, Zwane Mwaikambo wrote:
> On a slightly different topic, how come we're using physflat for hotplug
> cpu?
>
> -#ifndef CONFIG_CPU_HOTPLUG
> /* In the CPU hotplug case we cannot use broadcast mode
> because that opens a race when a CPU is removed.
> - Stay at physflat mode in this case.
> - It is bad to do this unconditionally though. Once
> - we have ACPI platform support for CPU hotplug
> - we should detect hotplug capablity from ACPI tables and
> - only do this when really needed. -AK */
> + Stay at physflat mode in this case. - AK */
> +#ifdef CONFIG_HOTPLUG_CPU
> if (num_cpus <= 8)
> genapic = &apic_flat;
What you say was true before this patch, (Although now that you point out i
realize the ifdef CONFIG_HOTPLUG_CPU is not required).
Think Andi is fixing this in his next drop to -mm*
When physflat was introduced, it also switched to use physflat mode for
#cpus <=8 when hotplug is enabled, since it doesnt use shortcuts and
so is also safer (although slower).
http://marc.theaimsgroup.com/?l=linux-kernel&m=112317686712929&w=2
The link above made using genapic_flat safer by using the
flat_send_IPI_mask(), and hence i switched back to using
logical flat mode when #cpus <=8, since that a little more efficient than
the send_IPI_mask_sequence() used in physflat mode.
In general we need
flat_mode - #cpus <= 8 (Hotplug defined or not, so we use mask version
for safety)
physflat or cluster_mode when #cpus >8.
If we choose physflat as default for #cpus <=8 (with hotplug) would make
IPI performance worse, since it would do one cpu at a time, and requires 2
writes per cpu for each IPI v.s just 2 for a flat mode mask version of the API.
--
Cheers,
Ashok Raj
- Open Source Technology Center
-
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