On Mon, May 19, 2014 at 03:57:37PM +0300, Alexander Shishkin wrote: > Peter Zijlstra <pet...@infradead.org> writes:
> > I'm not entirely thrilled to expose it to the PMU like this.. I realize > > you want this in order to get physically contiguous pages. > > Hmm, I guess we can have code in perf core to carry out the allocation > according to, say, contstraint flags and pass the page array down to the > PMU if that sounds like a cleaner thing to do? > > > Are you aware of allocation constraints for other architectures? > > Somewhat. ARM's trace memory controller supports both scatter-gather and > a plain contiguous buffer, I haven't found evidence of one being > available while the other one isn't, so I'm inclined to assume that if > it can write to system memory, it supports SG. I've just added a patch from Vince Weaver: http://lkml.kernel.org/r/alpine.deb.2.10.1405161708060.11...@vincent-weaver-1.umelst.maine.edu That adds pmu::capabilities, I suppose we could start with something like: PERF_PMU_CAP_AUX_BROKEN_SG which would make the allocator attempt to fill the AUX buffer with as big a chunks of contiguous memory as is available. > > That appears to be missing a is_power_of_2(aux_size) check. > > > > The problem with not having that is that since > > perf_event_mmap_page::aux_{head,tail} are of Z mod 2^64 but your actual > > {head,tail} are of Z mod aux_size, you need aux_size to be a full > > divider of 2^64 or otherwise you get wrapping issues at the overflow. > > > > Having it them all 2^n makes the divider trivial. > > I left it out so that the PMU callback could decide if it wants to do > the math or not. Maybe it can also be a constraint flag or is it not > worth it at all? I'd start with the most constrained model -- that is add the power of two test -- and worry about relaxing it if it turns out its really needed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
