Messages in this thread | | | Date | Mon, 6 Jan 2014 23:05:09 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v0 04/71] itrace: Infrastructure for instruction flow tracing units |
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On Mon, Jan 06, 2014 at 01:25:02PM -0800, Andi Kleen wrote: > Peter Zijlstra <peterz@infradead.org> 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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