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SubjectRe: [patch] add /proc/pid/stack to dump task's stack trace

* Alexey Dobriyan <adobriyan@gmail.com> wrote:

> On Fri, Nov 07, 2008 at 09:32:49AM +0100, Ingo Molnar wrote:
> >
> > * Alexey Dobriyan <adobriyan@gmail.com> wrote:
> >
> > > On Fri, Nov 07, 2008 at 08:59:25AM +0100, Ingo Molnar wrote:
> > > >
> > > > * Ingo Molnar <mingo@elte.hu> wrote:
> > > >
> > > > > > > + * buffer size used for proc read. See proc_info_read().
> > > > > > > + * 4K page size but our output routines use some slack for overruns
> > > > > > > + */
> > > > > > > +#define PROC_BLOCK_SIZE (3*1024)
> > > > >
> > > > > That sounds like a proper limit - the hard limit for this particular
> > > > > printout function is 4096-170, so we are well within the
> > > > > PROC_BLOCK_SIZE range.
> > > >
> > > > ok, i've added Ken's patch to tip/core/stacktrace and started testing
> > > > it.
> > > >
> > > > Alexey, i've added your Acked-by because you appeared to like the
> > > > patch - let me know if i should remove it.
> > >
> > > Of course, I don't like it!
> > >
> > > Switch to seqfiles, add entry in TID table as well.
> > >
> > > The idea is good, though.
> >
> > oh well - Ken, could you please switch it to seqfiles?
> >
> > It should be something like this to convert the currently sweet and
> > trivial function into a much more complex seqfile iterator splitup:
> >
> > - the ::start method does the kmalloc of a loop state structure like
> > this:
> >
> > {
> > struct stack_trace backtrace;
> > unsigned long entries[MAX_STACK_TRACE_DEPTH];
> >
> > int iterator;
> > }
> >
> > and saves the trace. (struct stack_trace trace can be on-stack, it's
> > only needed for the save_stack_trace() - and we keep the entries
> > after that.)
> >
> > - the ::stop method does the kfree of the loop state.
> >
> > - the ::show method prints a single line, based on ->entries[->iterator]
> >
> > - the ::next method does ->iterator++, and returns NULL if iterator
> > reaches ->backtrace.nr_entries.
> >
> > it will be more source code, larger kernel image, it will be more
> > fragile and will be harder to review, and it wont actually matter in
> > practice because 99.9999% of the backtraces we care about have a size
> > smaller than 3K. (and where they get larger clipping them to the first
> > 3K is perfectly fine)
>
> Or you can do all of this in ->show(), without start/next/stop:
>
> for (i = 0; i < N; i++)
> seq_printf(m, "[<%p>] %pS\n", x, x);

hm, is that preferred over the current fs/proc/base.c code?

If that's the preferred way of doing these things, why arent all the
single-page procfs functions converted over to seq_file:

#ifdef CONFIG_KALLSYMS
INF("wchan", S_IRUGO, pid_wchan),
#endif
+#ifdef CONFIG_STACKTRACE
+ INF("stack", S_IRUSR, pid_stack),
+#endif
#ifdef CONFIG_SCHEDSTATS
INF("schedstat", S_IRUGO, pid_schedstat),
#endif

?

Really, please realize what happened here. All this unnecessary work
comes from Ken just having done the _natural_ thing when extending the
existing upstream code: using the existing fs/proc/base.c methods as a
template to write new code. If those templates use outdated APIs, then
new code will be "outdated" too - and this confusion will go on
forever.

Ingo


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