On Tue 2018-11-06 23:35:02, Sergey Senozhatsky wrote:
> On (11/02/18 22:31), Tetsuo Handa wrote:
> > (1) Call get_printk_buffer() and acquire "struct printk_buffer *".
> >
> > (2) Rewrite printk() calls in the following way. The "ptr" is
> > "struct printk_buffer *" obtained in step (1).
> >
> > printk(fmt, ...) => printk_buffered(ptr, fmt, ...)
> > vprintk(fmt, args) => vprintk_buffered(ptr, fmt, args)
> > pr_emerg(fmt, ...) => bpr_emerg(ptr, fmt, ...)
> > pr_alert(fmt, ...) => bpr_alert(ptr, fmt, ...)
> > pr_crit(fmt, ...) => bpr_crit(ptr, fmt, ...)
> > pr_err(fmt, ...) => bpr_err(ptr, fmt, ...)
> > pr_warning(fmt, ...) => bpr_warning(ptr, fmt, ...)
> > pr_warn(fmt, ...) => bpr_warn(ptr, fmt, ...)
> > pr_notice(fmt, ...) => bpr_notice(ptr, fmt, ...)
> > pr_info(fmt, ...) => bpr_info(ptr, fmt, ...)
> > pr_cont(fmt, ...) => bpr_cont(ptr, fmt, ...)
> >
> > (3) Release "struct printk_buffer" by calling put_printk_buffer().
>
> [..]
>
> > Since we want to remove "struct cont" eventually, we will try to remove
> > both "implicit printk() users who are expecting KERN_CONT behavior" and
> > "explicit pr_cont()/printk(KERN_CONT) users". Therefore, converting to
> > this API is recommended.
>
> - The printk-fallback sounds like a hint that the existing 'cont' handling
> better stay in the kernel. I don't see how the existing 'cont' is
> significantly worse than
> bpr_warn(NULL, ...)->printk() // no 'cont' support
> I don't see why would we want to do it, sorry. I don't see "it takes 16
> printk-buffers to make a thing go right" as a sure thing.
I see it the following way:
+ mixed cont lines are very rare but they happen
+ 16 buffers are more than 1 so it could only be better [*]
+ the printk_buffer() code is self-contained and does not
complicate the logic of the classic printk() code [**]
[*] A missing put_printk_buffer() might cause that we would get
out of buffers. But the same problem is with locks,
disabled preemption, disabled interrupts, seq_buffer,
alloc/free. Such problems happen but they are rare.
Also I do not expect that the same buffer would be shared
between many functions. Therefore it should be easy
to use it correctly.
[**] I admit that cont buffer implementation is much easier
after removing the early flush to consoles but still...
Anyway, I do not think that both implementations are worth it.
We could keep both for some transition period but we should
remove the old one later.
> A question.
>
> How bad would it actually be to:
>
> - Allocate seq_buf 512-bytes buffer (GFP_ATOMIC) just-in-time, when we
> need it.
> // How often systems cannot allocate a 512-byte buffer? //
>
> - OK, assuming that systems around the world are so badly OOM like all the
> time and even kmalloc(512) is absolutely impossible, then have a fallback
> to the existing 'cont' handling; it just looks to me better than a plain
> printk()-fallback with removed 'cont' support.
This would prevent removing the fallback to struct cont. OOM is
one important scenario where continuous lines are used.
> - Do not allocate seq_buf if we are in printk-safe or in printk-nmi mode.
> To avoid "buffering for the sake of buffering". IOW, when in printk-safe
> use printk-safe.
Sure, my plan is to add a helper function is_buffered_printk_context() or so
that would check printk_context. Then we could do the following in
vprintk_buffered()
if (is_buffered_printk_context())
vprintk_func(....);
It might be added on top of the current patchset. I opened this
problem once and it got lost. So I did not want to complicate
it at this moment.
Best Regards,
Petr
