Messages in this thread |  | | Date | Mon, 8 May 2017 16:05:00 +0900 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH] printk: Add best-effort printk() buffering. |
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Hello,
sorry for the delay.
On (04/30/17 22:54), Tetsuo Handa wrote: > Sometimes we want to printk() multiple lines in a group without being > disturbed by concurrent printk() from interrupts and/or other threads. > For example, mixed printk() output of multiple thread's dump makes it > hard to interpret.
hm, it's very close to what printk-safe does [and printk-nmi, of course]. the difference is that buffered-printk does not disable local IRQs, unlike printk-safe, which has to do it by design. so the question is, can buffered-printk impose atomicity requirements? it seems that it can (am I wrong?). and, if so, then can we use printk-safe instead? we can add a new printk_buffered_begin/printk_buffered_end API, for example, (or enter/exit) for that purpose, that would set a buffered-printk `printk_context' bit so we can flush buffers in a "special way", not via IRQ work, and may be avoid message loss (printk-safe buffers are bigger in size than proposed PAGE_SIZE buffers).
> This patch introduces fixed-sized statically allocated buffers for > buffering printk() output for each thread/context in best effort > (i.e. up to PAGE_SIZE bytes, up to 16 concurrent printk() users).
hm, 16 is rather random, it's too much for UP and probably not enough for a 240 CPUs system. for the time being there are 3 buffered-printk users (as far as I can see), but who knows how more will be added in the future. each CPU can have overlapping printks from process, IRQ and NMI contexts. for NMI we use printk-nmi buffers, so it's out of the list; but, in general, *it seems* that we better depend on the number of CPUs the system has. which, once again, returns us back to printk-safe...
thoughts?
[..]
> +/* Must not be called from NMI context. */ > +static void __flush_printk_buffer(struct printk_buffer *ptr, bool all) > +{ > + unsigned long flags; > + > + if (!ptr->used) > + return; > + > + /* > + * Since printk_deferred() directly calls vprintk_emit(LOGLEVEL_SCHED), > + * this function does not need to care about LOGLEVEL_SCHED case. > + * Therefore, it is safe to call console_trylock() + console_unlock(). > + * > + * We don't call boot_delay_msec(level) here because level is unknown. > + */ > + printk_delay(); > + > + /* This stops the holder of console_sem just where we want him */ > + logbuf_lock_irqsave(flags); > + while (1) { > + char *text = ptr->buf; > + unsigned int text_len = ptr->used; > + char *cp = memchr(text, '\n', text_len); > + char c;
what guarantees that there'll always be a terminating newline?
-ss
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