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SubjectRe: [RFC PATCH v1 08/25] printk: add ring buffer and kthread
On (02/12/19 15:29), John Ogness wrote:
[..]
> +static int printk_kthread_func(void *data)
> +{
> + struct prb_iterator iter;
> + struct printk_log *msg;
> + size_t ext_len;
> + char *ext_text;
> + u64 master_seq;
> + size_t len;
> + char *text;
> + char *buf;
> + int ret;
> +
> + ext_text = kmalloc(CONSOLE_EXT_LOG_MAX, GFP_KERNEL);
> + text = kmalloc(PRINTK_SPRINT_MAX, GFP_KERNEL);
> + buf = kmalloc(PRINTK_RECORD_MAX, GFP_KERNEL);
> + if (!ext_text || !text || !buf)
> + return -1;
> +
> + prb_iter_init(&iter, &printk_rb, NULL);
> +
> + /* the printk kthread never exits */
> + for (;;) {
> + ret = prb_iter_wait_next(&iter, buf,
> + PRINTK_RECORD_MAX, &master_seq);
> + if (ret == -ERESTARTSYS) {
> + continue;
> + } else if (ret < 0) {
> + /* iterator invalid, start over */
> + prb_iter_init(&iter, &printk_rb, NULL);
> + continue;
> + }
> +
> + msg = (struct printk_log *)buf;
> + format_text(msg, master_seq, ext_text, &ext_len, text,
> + &len, printk_time);
> +
> + console_lock();
> + if (len > 0 || ext_len > 0) {
> + call_console_drivers(ext_text, ext_len, text, len);
> + boot_delay_msec(msg->level);
> + printk_delay();
> + }
> + console_unlock();
> + }

One thing that I have learned is that preemptible printk does not work
as expected; it wants to be 'atomic' and just stay busy as long as it can.
We tried preemptible printk at Samsung and the result was just bad:
preempted printk kthread + slow serial console = lots of lost messages

We also had preemptile printk in the upstream kernel and reverted the
patch (see fd5f7cde1b85d4c8e09); same reasons - we had reports that
preemptible printk could "stall" for minutes.

-ss

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