Re: [PATCH] printk: Fix panic log flush to serial console during kdump in PREEMPT_RT kernels
From: Ryo Takakura <hidden>
Date: 2025-08-19 13:31:56
Also in:
linux-rt-devel, lkml
Hi cuiguoqi! I'm not an expert on the subject, but hope this helps. On Thu, 7 Aug 2025 19:22:47 +0800, cuiguoqi wrote:
When a system running a real-time (PREEMPT_RT) kernel panics and triggers kdump, the critical log messages (e.g., panic reason, stack traces) may fail to appear on the serial console. When kdump cannot be used properly, serial console logs are crucial, whether for diagnosing kdump issues or troubleshooting the underlying problem.
The console not being flushed in the case of kexec should be expected as described [0]. Its about prioritizing kexec over serial output.
quoted hunk ↗ jump to hunk
This issue arises due to synchronization or deferred flushing of the printk buffer in real-time contexts, where preemptible console locks or delayed workqueues prevent timely log output before kexec transitions to the crash kernel. /** * kexec_image_info - For debugging output.@@ -176,6 +177,9 @@ void machine_kexec(struct kimage *kimage)pr_info("Bye!\n"); + if (IS_ENABLED(CONFIG_PREEMPT_RT) && in_kexec_crash) + console_flush_on_panic(CONSOLE_FLUSH_PENDING); +
Calling console_flush_on_panic() while trying to kexec will reduce its chance of success.
quoted hunk ↗ jump to hunk
diff --git a/kernel/panic.c b/kernel/panic.c index 72fcbb5..e0ad0df 100644 --- a/kernel/panic.c +++ b/kernel/panic.c@@ -437,6 +437,8 @@ void vpanic(const char *fmt, va_list args)*/ kgdb_panic(buf); + printk_legacy_allow_panic_sync(); + /* * If we have crashed and we have a crash kernel loaded let it handle * everything else.@@ -450,8 +452,6 @@ void vpanic(const char *fmt, va_list args)panic_other_cpus_shutdown(_crash_kexec_post_notifiers); - printk_legacy_allow_panic_sync(); -
The ordering here should be kept where we don't want CPUs other than the one panicked flushing legacy consoles. Sincerely, Ryo Takakura [0] https://lore.kernel.org/lkml/847cagmjsx.fsf@jogness.linutronix.de/ (local)