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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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