Re: [PATCH v1] printk: Add CON_NO_SUPPRESS console flag and use it for ramoops

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 06-12 17:26, Pasha Tatashin wrote:
> The ramoops console is designed to capture and preserve logs across warm
> reboots and liveupdates. It is registered during boot and captures logs
> from that point forward. However, if a 'quiet' loglevel is specified on
> the kernel command line, ramoops console logs (including important
> shutdown/kexec handover logs) can be filtered out and suppressed by
> printk.
> 
> Specifying 'quiet' on the kernel command line is highly desirable to
> avoid severe warm-reboot slowdowns, since physical serial consoles are
> typically bottlenecked by slow baud rates. However, we still need to
> fully capture all logs in memory via ramoops without suppression.
> 
> Introduce a generic console flag CON_NO_SUPPRESS to allow specific
> consoles (like the ramoops/pstore console) to opt-out of loglevel
> suppression. Check this flag safely in console_emit_next_record() and
> nbcon_emit_next_record(). Finally, set this flag on registration of the
> pstore console in pstore_register_console().
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>

This patch is not needed base on the discussion in:
https://lore.kernel.org/all/CAAi7L5fZTvhb5np=xGWnDEmr4bNXqoAcMhR_ePZ4Qga4E-KucQ@mail.gmail.com

> ---
>  fs/pstore/platform.c    | 2 +-
>  include/linux/console.h | 3 +++
>  kernel/printk/nbcon.c   | 5 ++++-
>  kernel/printk/printk.c  | 4 +++-
>  4 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
> index 1d76c9d92056..9efbaa9895da 100644
> --- a/fs/pstore/platform.c
> +++ b/fs/pstore/platform.c
> @@ -420,7 +420,7 @@ static void pstore_register_console(void)
>  	 * Always initialize flags here since prior unregister_console()
>  	 * calls may have changed settings (specifically CON_ENABLED).
>  	 */
> -	pstore_console.flags = CON_PRINTBUFFER | CON_ENABLED | CON_ANYTIME;
> +	pstore_console.flags = CON_PRINTBUFFER | CON_ENABLED | CON_ANYTIME | CON_NO_SUPPRESS;
>  	register_console(&pstore_console);
>  }
>  
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 5520e4477ad7..9bd06d3fece3 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -180,6 +180,8 @@ static inline void con_debug_leave(void) { }
>   *			constraints.
>   * @CON_NBCON_ATOMIC_UNSAFE: The write_atomic() callback is not safe and is
>   *			therefore only used by nbcon_atomic_flush_unsafe().
> + * @CON_NO_SUPPRESS:	Indicates that the console is exempt from loglevel-based
> + *			message suppression (e.g. from the 'quiet' boot argument).
>   */
>  enum cons_flags {
>  	CON_PRINTBUFFER		= BIT(0),
> @@ -192,6 +194,7 @@ enum cons_flags {
>  	CON_SUSPENDED		= BIT(7),
>  	CON_NBCON		= BIT(8),
>  	CON_NBCON_ATOMIC_UNSAFE	= BIT(9),
> +	CON_NO_SUPPRESS		= BIT(10),
>  };
>  
>  /**
> diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
> index d7044a7a214b..cd0fc5f42360 100644
> --- a/kernel/printk/nbcon.c
> +++ b/kernel/printk/nbcon.c
> @@ -989,6 +989,7 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
>  	struct nbcon_state cur;
>  	unsigned long dropped;
>  	unsigned long ulseq;
> +	bool may_suppress;
>  
>  	/*
>  	 * This function should never be called for consoles that have not
> @@ -1014,7 +1015,9 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
>  	if (!nbcon_context_enter_unsafe(ctxt))
>  		return false;
>  
> -	ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended, true);
> +	may_suppress = !(console_srcu_read_flags(con) & CON_NO_SUPPRESS);
> +	ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended,
> +						may_suppress);
>  	if (!ctxt->backlog)
>  		return nbcon_context_exit_unsafe(ctxt);
>  
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 0323149548f6..f6502cb84160 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3131,10 +3131,12 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
>  		.pbufs = &printk_shared_pbufs,
>  	};
>  	unsigned long flags;
> +	bool may_suppress;
>  
>  	*handover = false;
>  
> -	if (!printk_get_next_message(&pmsg, con->seq, is_extended, true))
> +	may_suppress = !(console_srcu_read_flags(con) & CON_NO_SUPPRESS);
> +	if (!printk_get_next_message(&pmsg, con->seq, is_extended, may_suppress))
>  		return false;
>  
>  	con->dropped += pmsg.dropped;
> -- 
> 2.53.0
>