Re: [PATCH 2/2] mm: add a field to store names for private anonymous memory

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 12, 2013 at 11:45:17AM +0200, Ingo Molnar wrote:
> One thing where we could help JITs is to offer a direct channel to any 
> perf profiling process: a prctl(SYS_TRACE) which would send a free-form 
> string to any profiling task interested in it.
> 
> This would be a glorified anonymous write() in essence, without using a 
> temporary file.
> 
> The advantage would be that the string could be captured as-is and copied 
> to the ring-buffer of the profiling task - instead of having to recover it 
> later on.
> 
> This is a model that I'd generally advocate: a single channel [per 
> CPU-ified] for instrumentation/tracing.

'free format text string' is long and cumbersome and requires parsing.

And size is the primary component in speed.

But yes, we could allow injection of something like 

struct PERF_RECORD_SYMBOL {
	struct perf_event_header	header;
	u32				pid, tid;
	u64				addr;
	u64				len;
	char				symbol[];
};

I still like the idea of actually writing valid ELF DSOs in that that would
also get us the TEXT and allow assembly inspection etc. It might also allow a
JIT to re-map those DSOs and decrease warm-up time -- provided the actual
program didn't change meanwhile.

How to do injection is another thing though; I don't much like prctl(). Then
again, offering a special file like /sys/bus/event_source/sink isn't
particularly pretty either.

Then there is the issue of attaching to an already running JIT; we'd need means
to 'catch' up. The DSOs trivially allow this; the injection not so much.

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