On Wed, Nov 02, 2016 at 05:05:47PM +0100, Sebastian Andrzej Siewior wrote:
> On 2016-11-01 05:31:56 [+0200], Michael S. Tsirkin wrote:
> > > > > > >
> > > > > > > > - optionally it has been requested to make synchronize_rcu() behave like
> > > > > > > > synchronize_rcu_expedited() on shutdown and kexec().
> >
> > I was trying to say that the fact net core calls expedited sync
> > implies that many people care about boot time.
> >
> > Should rcu_normal_after_boot override rcu_expedited?
> > I'm not sure what happens if you specify both ATM.
>
> I think what you asked for is something like this:
>
> diff --git a/kernel/reboot.c b/kernel/reboot.c
> --- a/kernel/reboot.c
> +++ b/kernel/reboot.c
> @@ -283,6 +283,7 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
> struct pid_namespace *pid_ns = task_active_pid_ns(current);
> char buffer[256];
> int ret = 0;
> + bool force_rcu_exp = false;
>
> /* We only trust the superuser with rebooting the system. */
> if (!ns_capable(pid_ns->user_ns, CAP_SYS_BOOT))
> @@ -311,7 +312,19 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
> if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)
> cmd = LINUX_REBOOT_CMD_HALT;
>
> + switch (cmd) {
> + case LINUX_REBOOT_CMD_RESTART:
> + case LINUX_REBOOT_CMD_HALT:
> + case LINUX_REBOOT_CMD_POWER_OFF:
> + case LINUX_REBOOT_CMD_RESTART2:
> + case LINUX_REBOOT_CMD_KEXEC:
> + force_rcu_exp = true;
> + }
> +
> mutex_lock(&reboot_mutex);
> + if (force_rcu_exp)
> + rcu_expedite_gp();
> +
> switch (cmd) {
> case LINUX_REBOOT_CMD_RESTART:
> kernel_restart(NULL);
> @@ -362,6 +375,8 @@ SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
> ret = -EINVAL;
> break;
> }
> + if (force_rcu_exp)
> + rcu_unexpedite_gp();
> mutex_unlock(&reboot_mutex);
> return ret;
> }
>
> and problem that this patch is pointless because it is too late. This
> RESTART, POWER_OFF, KEXEC hooks here called once the `shutdown' binary
> is done doing sync & cleanup and invokes the kernel for the final
> operation.
> So to achieve what you ask for, we would need a
> LINUX_REBOOT_CMD_RCU_FAST option which is invoked by the `shutdown'
> binary as the first thing - long before it gets to
> LINUX_REBOOT_CMD_RESTART (or KEXEC).
FWIW, this looks sane to me.
Thanx, Paul
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