On Sat 2018-11-10 17:52:17, Tetsuo Handa wrote:
> On 2018/11/10 0:43, Petr Mladek wrote:
> > On Fri 2018-11-09 18:55:26, Tetsuo Handa wrote:
> >> How early_printk requirement affects line buffered printk() API?
> >>
> >> I don't think it is impossible to convert from
> >>
> >> printk("Testing feature XYZ..");
> >> this_may_blow_up_because_of_hw_bugs();
> >> printk(KERN_CONT " ... ok\n");
> >>
> >> to
> >>
> >> printk("Testing feature XYZ:\n");
> >> this_may_blow_up_because_of_hw_bugs();
> >> printk("Testing feature XYZ.. ... ok\n");
> >>
> >> in https://lore.kernel.org/lkml/CA+55aFwmwdY_mMqdEyFPpRhCKRyeqj=+aCqe5nN108v8ELFvPw@xxxxxxxxxxxxxx/ .
> >
> > I just wonder how this pattern is common. I have tried but I failed
> > to find any instance.
> >
> > This problem looks like a big argument against explicit buffers.
> > But I wonder if it is real.
>
> An example of boot up messages where buffering makes difference.
>
> Vanilla:
>
> [ 0.260459] smp: Bringing up secondary CPUs ...
> [ 0.269595] x86: Booting SMP configuration:
> [ 0.270461] .... node #0, CPUs: #1
> [ 0.066578] Disabled fast string operations
> [ 0.066578] mce: CPU supports 0 MCE banks
> [ 0.066578] smpboot: CPU 1 Converting physical 2 to logical package 1
> [ 0.342569] #2
> [ 0.066578] Disabled fast string operations
> [ 0.066578] mce: CPU supports 0 MCE banks
> [ 0.066578] smpboot: CPU 2 Converting physical 4 to logical package 2
> [ 0.413442] #3
> [ 0.066578] Disabled fast string operations
> [ 0.066578] mce: CPU supports 0 MCE banks
> [ 0.066578] smpboot: CPU 3 Converting physical 6 to logical package 3
> [ 0.476562] smp: Brought up 1 node, 4 CPUs
> [ 0.477477] smpboot: Max logical packages: 8
> [ 0.477514] smpboot: Total of 4 processors activated (22691.70 BogoMIPS)
>
> With try_buffered_printk() patch:
>
> [ 0.279768] smp: Bringing up secondary CPUs ...
> [ 0.288825] x86: Booting SMP configuration:
> [ 0.066748] Disabled fast string operations
> [ 0.066748] mce: CPU supports 0 MCE banks
> [ 0.066748] smpboot: CPU 1 Converting physical 2 to logical package 1
> [ 0.066748] Disabled fast string operations
> [ 0.066748] mce: CPU supports 0 MCE banks
> [ 0.066748] smpboot: CPU 2 Converting physical 4 to logical package 2
> [ 0.066748] Disabled fast string operations
> [ 0.066748] mce: CPU supports 0 MCE banks
> [ 0.066748] smpboot: CPU 3 Converting physical 6 to logical package 3
> [ 0.495862] .... node #0, CPUs: #1 #2 #36smp: Brought up 1 node, 4 CPUs
> [ 0.496833] smpboot: Max logical packages: 8
> [ 0.497609] smpboot: Total of 4 processors activated (22665.22 BogoMIPS)
>
>
>
> Hmm, arch/x86/kernel/smpboot.c is not emitting '\n' after #num
>
> if (system_state < SYSTEM_RUNNING) {
> if (node != current_node) {
> if (current_node > (-1))
> pr_cont("\n");
> current_node = node;
>
> printk(KERN_INFO ".... node %*s#%d, CPUs: ",
> node_width - num_digits(node), " ", node);
> }
>
> /* Add padding for the BSP */
> if (cpu == 1)
> pr_cont("%*s", width + 1, " ");
>
> pr_cont("%*s#%d", width - num_digits(cpu), " ", cpu);
>
> } else
> pr_info("Booting Node %d Processor %d APIC 0x%x\n",
> node, cpu, apicid);
>
> and causing
>
> pr_info("Brought up %d node%s, %d CPU%s\n",
> num_nodes, (num_nodes > 1 ? "s" : ""),
> num_cpus, (num_cpus > 1 ? "s" : ""));
>
> line to be concatenated to previous line.
> Maybe disable try_buffered_printk() if system_state !=
> SYSTEM_RUNNING ?
We need to solve continuous lines also during boot. Also similar
problems might be in the code that is called in SYSTEM_RUNNING state.
This is yet another clue that try_buffered_printk() approach is not
that good idea.
Best Regards,
Petr
