linux-kernel - Re: [PATCH] printk: Add best-effort printk() buffering.

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Date:   Tue, 9 May 2017 10:04:42 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     sergey.senozhatsky.work@...il.com, linux-kernel@...r.kernel.org,
        mingo@...hat.com, peterz@...radead.org, pmladek@...e.com,
        sergey.senozhatsky@...il.com, rostedt@...dmis.org
Subject: Re: [PATCH] printk: Add best-effort printk() buffering.

Hello,

On (05/08/17 22:05), Tetsuo Handa wrote:
> > 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).
> 
> printk_buffered_begin()/printk_buffered_end() corresponds to
> get_printk_buffer()/put_printk_buffer().
> printk_context() distinguishes atomic contexts.
> flush_printk_buffer() flushes from non-NMI context.
> 
> What does atomicity requirements mean?

what I meant was -- "can we sleep under printk_buffered_begin() or not".
printk-safe disables local IRQs. so what I propose is something like this

	printk-safe-enter    //disable local IRQs, use per-CPU buffer
	backtrace
	printk-safe-exit     //flush per-CPU buffer, enable local IRQs

except that 'printk-safe-enter/exit' will have new names here, say
printk-buffered-begin/end, and, probably, handle flush differently.


> > 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?
> 
> I can make 16 a CONFIG_ option.

but still, why use additional N buffers, when we already have per-CPU
buffers? what am I missing?

> Would you read 201705031521.EIJ39594.MFtOVOHSFLFOJQ@...ove.SAKURA.ne.jp ?

sure.

> But as long as actually writing to console devices is slow, message loss
> is inevitable no matter how big buffer is used. Rather, I'd expect an API
> which allows printk() users in schedulable context (e.g. kmallocwd and/or
> warn_alloc() for reporting allocation stalls) to wait until written to
> console devices. That will more likely to reduce message loss.

hm, from a schedulable context you can do *something* like

	console_lock()
	printk()
	...
	printk()
	console_unlock()


you won't be able to console_lock() until all pending messages are
flushed. since you are in a schedulable context, you can sleep on
console_sem in console_lock(). well, just saying.


> > > +	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?
> 
> Nothing guarantees. Why need such a guarantee?

 : The memchr() and memrchr() functions return a pointer to the matching
 : byte or NULL if the character does not occur in the given memory area.


so `cp' can be NULL here?

+               if (cp++)
+                       text_len = cp - text;
+               else if (all)
+                       cp = text + text_len;
+               else
+                       break;
+               /* printk_get_level() depends on text '\0'-terminated. */
+               c = *cp;
+               *cp = '\0';
+               process_log(0, LOGLEVEL_DEFAULT, NULL, 0, text, text_len);
+               ptr->used -= text_len;
+               if (!ptr->used)
+                       break;
+               *cp = c;

	-ss