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 ?
