From: Sebastian Andrzej Siewior Date: Thu, 21 Mar 2013 19:01:05 +0100 Subject: printk: Drop the logbuf_lock more often Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.1-rt3.tar.xz The lock is hold with irgs off. The latency drops 500us+ on my arm bugs with a "full" buffer after executing "dmesg" on the shell. Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/printk.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1415,12 +1415,23 @@ static int syslog_print_all(char __user u64 next_seq; u64 seq; u32 idx; + int attempts = 0; + int num_msg; text = kmalloc(LOG_LINE_MAX + PREFIX_MAX, GFP_KERNEL); if (!text) return -ENOMEM; logbuf_lock_irq(); + +try_again: + attempts++; + if (attempts > 10) { + len = -EBUSY; + goto out; + } + num_msg = 0; + /* * Find first record that fits, including all following records, * into the user-provided buffer for this dump. @@ -1433,6 +1444,14 @@ static int syslog_print_all(char __user len += msg_print_text(msg, true, NULL, 0); idx = log_next(idx); seq++; + num_msg++; + if (num_msg > 5) { + num_msg = 0; + logbuf_unlock_irq(); + logbuf_lock_irq(); + if (clear_seq < log_first_seq) + goto try_again; + } } /* move first record forward until length fits into the buffer */ @@ -1444,6 +1463,14 @@ static int syslog_print_all(char __user len -= msg_print_text(msg, true, NULL, 0); idx = log_next(idx); seq++; + num_msg++; + if (num_msg > 5) { + num_msg = 0; + logbuf_unlock_irq(); + logbuf_lock_irq(); + if (clear_seq < log_first_seq) + goto try_again; + } } /* last message fitting into this dump */ @@ -1481,6 +1508,7 @@ static int syslog_print_all(char __user clear_seq = log_next_seq; clear_idx = log_next_idx; } +out: logbuf_unlock_irq(); kfree(text);