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


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