Re: [patch 1/2] x86_64 page fault NMI-safe

From: Peter Zijlstra
Date: Wed Aug 04 2010 - 10:56:57 EST

On Wed, 2010-08-04 at 10:45 -0400, Mathieu Desnoyers wrote:

> How do you plan to read the data concurrently with the writer overwriting the
> data while you are reading it without corruption ?

I don't consider reading while writing (in overwrite mode) a valid case.

If you want to use overwrite, stop the writer before reading it.

> I think that the stack dump
> should simply be saved directly to the ring buffer, without copy. The
> dump_stack() functions might have to be extended so they don't just save text
> dumbly, but can also be used to save events into the trace in binary format,
> perhaps with the continuation cookie Linus was proposing.

Because I don't want to support truncating reservations (because that
leads to large nops for nested events) and when the event needs to go to
multiple buffers you can re-use the stack-dump without having to do the
unwind again.

The problem with the continuation thing Linus suggested is that it would
bloat the output 3 fold. A stack entry is a single u64. If you want to
wrap that in a continuation event you need: a header (u64), a cookie
(u64) and the entry (u64).

Continuation events might make heaps of sense for larger data pieces,
but I don't see them being practical for such small pieces.
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