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SubjectRe: [PATCH] printk: inject caller information into the body of message
On (09/14/18 19:37), Tetsuo Handa wrote:
> > @@ -20,6 +20,9 @@
> > * Annotation for a "continued" line of log printout (only done after a
> > * line that had no enclosing \n). Only to be used by core/arch code
> > * during early bootup (a continued line is not SMP-safe otherwise).
> > + *
> > + * Please consider pr_line()/vpr_line() functions for SMP-safe continued
> > + * line printing.
>
> I think the advantage is not limited to SMP-safeness. Reducing the frequency of
> calling printk() will reduce overhead. Also, latency for netconsole will be
> reduced by sending a whole line in one printk().

Hmm. These are very good points, indeed. But do we want to list all
advantages here? I just wanted to mention SMP-unsafe pr_cont/printk(KERN_CONT),
because I also mention pr_line in kern_levels.h.

> > + * Defines a new pr_line varialbe, which would use an implicit
>
> s/varialbe/variable/ .

Thanks.

> > +#define DEFINE_PR_LINE(lev, name) \
> > + char __line_##name[__PR_LINE_BUF_SZ]; \
> > + struct pr_line name = { \
> > + .sb = __SEQ_BUF_INITIALIZER(__line_##name, \
> > + __PR_LINE_BUF_SZ), \
> > + .level = lev, \
> > + }
>
> Want a note that
>
> static DEFINE_PR_LINE(lev, name);
>
> won't make "name" variable "static" ?

Interesting point. Any hint what the comment should look like?
Do we want to have static pr_line buffers?

> > +#define DEFINE_PR_LINE_BUF(lev, name, buf, sz) \
> > + struct pr_line name = { \
> > + .sb = __SEQ_BUF_INITIALIZER(buf, (sz)), \
> > + .level = lev, \
> > + }
> > +
>
> I would use this one for the OOM killer. 80 bytes is too short.

80 bytes is quite short for OOM, agreed.

> static char oom_print_buf[1024];
> DEFINE_PR_LINE_BUF(level, oom_print_buf);

Do I get it right that you suggest to drop the "size" param?
Do OOM people agree on 1024 bytes stack usage?


> > @@ -131,4 +187,8 @@ extern int
> > seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
> > #endif
> >
> > +extern __printf(2, 0)
> > +int vpr_line(struct pr_line *pl, const char *fmt, va_list args);
> > +extern __printf(2, 3)
> > +int pr_line(struct pr_line *pl, const char *fmt, ...);
>
> Do we want to mark "asmlinkage" like printk() ?

Dunno, do we? Does code written in assembly call pr_cont that often?
We are not turning pr_line() into syscall anyway.

> > @@ -324,3 +324,60 @@ int seq_buf_to_user(struct seq_buf *s, char __user *ubuf, int cnt)
> > s->readpos += cnt;
> > return cnt;
> > }
> > +
> > +/**
> > + * vpr_line - Append data to the printk() line buffer
> > + * @pl: the pr_line descriptor
>
> s/descriptor/structure/ ?

Yeah, I used the term "descriptor", just because it's used in seq_buf.c.
So, it's sort of common in seq_buf.
E.g.
seq_buf_vprintf(), seq_buf_print_seq(), seq_buf_can_fit() and so on.

> > + * @fmt: printf format string
> > + * @args: va_list of arguments from a printf() type function
> > + *
> > + * Writes a vnprintf() format into the printk() pr_line buffer.
>
> s/vnprintf/vprintf/ ?

Indeed.
We also need to fix a typo in seq_buf_vprintf() comment then.

-ss

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