Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flowtracing units

[Peter Zijlstra]

On Mon, Jan 06, 2014 at 01:25:02PM -0800, Andi Kleen wrote:
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
> 
> > On Thu, Dec 19, 2013 at 04:30:53PM +0200, Alexander Shishkin wrote:
> >> So I'd like to steer away from the ways in which hardware can be broken
> >> and talk about a usable interface, to begin with.
> >
> > Just dump it into the regular one buffer like I outlined.
> 
> Just getting back to this. 
> 
> Do you realize that PT buffers have to be page aligned. 
> 
> So to mix it with a regular perf buffer would need padding every PT
> message by 4K, which wastes a lot of memory. The side band messages
> are usually only a few bytes (e.g. context switch).
> 
> If the sideband is mfrequent it could even take up >half of the buffer,
> but mostly only with padding.
> 
> Is that what you intended?
> 
> perf doesn't support gaps today, so your proposal wouldn't even
> seem to fit into the current perf design.

That would a really trivial addition.

> Also of course it requires disabling/enabling PT explicitly for 
> every perf message, which is slow. So you add at least 2*WRMSR cost
> (thousands of cycles).

That's just dumb, no flush the entire PT buffer into a few large
records.

> > That said; we very much need to have at least two architectures
> > implemented for any of this code to move.
> >
> > But we cannot ignore the hardware trainwreck; we cannot shape our
> > interface around something that's utterly broken.
> >
> > Some hardware is just too broken to support.
> 
> I don't think the PT design is broken in any way, it's straight 
> forward and simple.

If it were actually implemented like the spec says and not have this
crappy S/G limitation, then maybe.

> Trying to mix hardware tracing and software tracing in the same buffer
> on the other hand ...
> 
> Anyways if perf is not flexible enough to support this I suppose
> it could switch to a simple device driver, and only run perf with
> separate fds for side band purposes. 
> 
> Would you prefer that?

Don't be stupid.
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