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authorRusty Russell <rusty@rustcorp.com.au>2012-08-06 17:29:49 +0930
committerIngo Molnar <mingo@kernel.org>2012-08-23 10:45:13 +0200
commit816afe4ff98ee10b1d30fd66361be132a0a5cee6 (patch)
treeedb054530d87da95a9a9054d50467d92b62d8214 /arch/x86/xen/smp.c
parent23dcfa61bac244e1200ff9ad19c6e9144dcb6bb5 (diff)
x86/smp: Don't ever patch back to UP if we unplug cpus
We still patch SMP instructions to UP variants if we boot with a single CPU, but not at any other time. In particular, not if we unplug CPUs to return to a single cpu. Paul McKenney points out: mean offline overhead is 6251/48=130.2 milliseconds. If I remove the alternatives_smp_switch() from the offline path [...] the mean offline overhead is 550/42=13.1 milliseconds Basically, we're never going to get those 120ms back, and the code is pretty messy. We get rid of: 1) The "smp-alt-once" boot option. It's actually "smp-alt-boot", the documentation is wrong. It's now the default. 2) The skip_smp_alternatives flag used by suspend. 3) arch_disable_nonboot_cpus_begin() and arch_disable_nonboot_cpus_end() which were only used to set this one flag. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> Cc: Paul McKenney <paul.mckenney@us.ibm.com> Cc: Suresh Siddha <suresh.b.siddha@intel.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/r/87vcgwwive.fsf@rustcorp.com.au Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/xen/smp.c')
-rw-r--r--arch/x86/xen/smp.c6
1 files changed, 2 insertions, 4 deletions
diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index f58dca7a6e5..353c50f1870 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -377,7 +377,8 @@ static int __cpuinit xen_cpu_up(unsigned int cpu, struct task_struct *idle)
return rc;
if (num_online_cpus() == 1)
- alternatives_smp_switch(1);
+ /* Just in case we booted with a single CPU. */
+ alternatives_enable_smp();
rc = xen_smp_intr_init(cpu);
if (rc)
@@ -424,9 +425,6 @@ static void xen_cpu_die(unsigned int cpu)
unbind_from_irqhandler(per_cpu(xen_irq_work, cpu), NULL);
xen_uninit_lock_cpu(cpu);
xen_teardown_timer(cpu);
-
- if (num_online_cpus() == 1)
- alternatives_smp_switch(0);
}
static void __cpuinit xen_play_dead(void) /* used only with HOTPLUG_CPU */