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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
