Messages in this thread | | | Date | Mon, 15 Sep 2008 17:14:34 -0400 (EDT) | From | Steven Rostedt <> | Subject | Re: Tracing/ftrace: trouble with trace_entries and trace_pipe |
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On Mon, 15 Sep 2008, Pekka Paalanen wrote:
> Hi Steven and others, > > first a minor bug: it seems the units of /debug/tracing/trace_entries > is different for read and write. This is confusing for the users, since > I can't say "if you have problems, double it". If I read from it > something like 16422, then write back 16422, next I read 214. I can't > recall the exact numbers, but the difference really is two orders of > magnitude. I have 1 GB RAM in this box, so that shouldn't be an issue.
You write to it the same number that you read from and it returned something different?? That is indeed a bug, since it should definitely detect that. Is this linux-tip? I'll have to play with it to make sure nothing broke it recently.
I just tried the latest mainline, and it seemed to work there.
> > My other problem is with trace_pipe. It is again making 'cat' quit too > early. The condition triggered is > if (!tracer_enabled && iter->pos) { > in tracing_read_pipe(), and it is followed by triggering > /* stop when tracing is finished */ > if (trace_empty(iter)) { > and then sret=0, so read returns 0 and 'cat' exits. > > Now, I am trying my mmiotrace marker patches, but as far as I can tell, > nothing I modified is the reason for this. I didn't yet explicitly test > for it, though. I'll send these patches after I hear from Frederic. > > The cat-quit problem is not a constant state. After boot, I could play > with my markers and testmmiotrace without cat quitting. Then something > happens, and cat starts the quitting behaviour, and won't get to normal > by disabling and enabling mmiotrace. > > I have a couple of wild guesses of what might be related: > - ring buffer wrap-around > - ring buffer overflow (at first try I hit these, the second try > after putting debug-pr_info's in place I don't hit this) > - ring buffer resize (after playing with trace_entries, cat-quit > problem was present, though it might have been present before) > > After viewing the git history, I have some more guesses, mainly > related to setting tracer_enabled to 0. > - commit 2b1bce1787700768cbc87c8509851c6f49d252dc > I don't see where tracer_enabled would be set to 1, when > mmiotrace is enabled. It used to default to 1 and mmiotrace was happy. > - __tracing_open() sets it to 0 (not called for the pipe) > - tracing_release() sets it to 1 > - tracing_ctrl_write() toggles it > - tracing_read_pipe() tests it > - tracer_alloc_buffers() uses it > And other tracers seem to use it a lot. > > Mmiotrace does use the tracer::ctrl_update hook, and allow/disallow > calls to __trace_mmiotrace_{rw,map}() via enabling/disabling the whole > mmiotrace core. Is this not enough, or is it inappropriate? > > It seems tracer_enabled is used by the trace framework itself to > enable/disable... what? Hmm, maybe nothing I care about. > > Should mmiotrace simply do > tracer_enabled = 1; > in mmio_trace_init()? > > Should mmiotrace test tracer_enabled, and if so, when?
No, tracer_enabled is something that is internal to the tracer infrastructure.
if you read from either tracing/trace or tracing/latency_trace it will disable the tracer while you dump. But you should not be doing that. The trace_pipe should not disable that either.
-- Steve
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