On Fri, 2012-07-27 at 17:08 +0930, Rusty Russell wrote:
> 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.
I am ok with this proposal. If I follow correctly, you are still
allowing the patching to UP happen on boot if there is only one online
cpu currently but a possiblity of multiple cpu's that can come online
later. Perhaps make this little more explicit in the changelog.
Also,
> + /* Patch to UP if other cpus not imminent. */
> + if (noreplace_smp || num_present_cpus() == 1 || setup_max_cpus <= 1) {
shouldn't this be
if (!noreplace_smp && (num_present_cpus() == 1 || setup_max_cpus <= 1))
also, to be consistent with other checks, may be just use
"num_possible_cpus() == 1" check instead of "setup_max_cpus <= 1".
thanks,
suresh
> + uniproc_patched = true;
> alternatives_smp_module_add(NULL, "core kernel",
> __smp_locks, __smp_locks_end,
> _text, _etext);
> -
> - /* Only switch to UP mode if we don't immediately boot others */
> - if (num_present_cpus() == 1 || setup_max_cpus <= 1)
> - alternatives_smp_switch(0);
> }
> -#endif
> apply_paravirt(__parainstructions, __parainstructions_end);
>
> - if (smp_alt_once)
> + if (!uniproc_patched || num_possible_cpus() == 1)
> free_init_pages("SMP alternatives",
> (unsigned long)__smp_locks,
> (unsigned long)__smp_locks_end);
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -665,7 +665,8 @@ static int __cpuinit do_boot_cpu(int api
> unsigned long boot_error = 0;
> int timeout;
>
> - alternatives_smp_switch(1);
> + /* Just in case we booted with a single CPU. */
> + alternatives_enable_smp();
>
> idle->thread.sp = (unsigned long) (((struct pt_regs *)
> (THREAD_SIZE + task_stack_page(idle))) - 1);
> @@ -1053,20 +1054,6 @@ out:
> preempt_enable();
> }
>
> -void arch_disable_nonboot_cpus_begin(void)
> -{
> - /*
> - * Avoid the smp alternatives switch during the disable_nonboot_cpus().
> - * In the suspend path, we will be back in the SMP mode shortly anyways.
> - */
> - skip_smp_alternatives = true;
> -}
> -
> -void arch_disable_nonboot_cpus_end(void)
> -{
> - skip_smp_alternatives = false;
> -}
> -
> void arch_enable_nonboot_cpus_begin(void)
> {
> set_mtrr_aps_delayed_init();
> @@ -1256,9 +1243,6 @@ void native_cpu_die(unsigned int cpu)
> if (per_cpu(cpu_state, cpu) == CPU_DEAD) {
> if (system_state == SYSTEM_RUNNING)
> pr_info("CPU %u is now offline\n", cpu);
> -
> - if (1 == num_online_cpus())
> - alternatives_smp_switch(0);
> return;
> }
> msleep(100);
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -439,14 +439,6 @@ EXPORT_SYMBOL_GPL(cpu_up);
> #ifdef CONFIG_PM_SLEEP_SMP
> static cpumask_var_t frozen_cpus;
>
> -void __weak arch_disable_nonboot_cpus_begin(void)
> -{
> -}
> -
> -void __weak arch_disable_nonboot_cpus_end(void)
> -{
> -}
> -
> int disable_nonboot_cpus(void)
> {
> int cpu, first_cpu, error = 0;
> @@ -458,7 +450,6 @@ int disable_nonboot_cpus(void)
> * with the userspace trying to use the CPU hotplug at the same time
> */
> cpumask_clear(frozen_cpus);
> - arch_disable_nonboot_cpus_begin();
>
> printk("Disabling non-boot CPUs ...\n");
> for_each_online_cpu(cpu) {
> @@ -474,8 +465,6 @@ int disable_nonboot_cpus(void)
> }
> }
>
> - arch_disable_nonboot_cpus_end();
> -
> if (!error) {
> BUG_ON(num_online_cpus() > 1);
> /* Make sure the CPUs won't be enabled by someone else */
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