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Date: Tue, 28 May 2013 11:54:25 +0200 From: Ingo Molnar <mingo@...nel.org> To: Stephane Eranian <eranian@...gle.com> Cc: Peter Zijlstra <peterz@...radead.org>, Jiri Olsa <jolsa@...hat.com>, LKML <linux-kernel@...r.kernel.org>, Corey Ashford <cjashfor@...ux.vnet.ibm.com>, Frederic Weisbecker <fweisbec@...il.com>, Ingo Molnar <mingo@...e.hu>, Namhyung Kim <namhyung@...nel.org>, Paul Mackerras <paulus@...ba.org>, Arnaldo Carvalho de Melo <acme@...hat.com>, Andi Kleen <ak@...ux.intel.com>, David Ahern <dsahern@...il.com> Subject: Re: [PATCH 0/9] perf: Adding better precise_ip field handling * Stephane Eranian <eranian@...gle.com> wrote: > On Mon, May 13, 2013 at 9:43 PM, Ingo Molnar <mingo@...nel.org> wrote: > > > > * Peter Zijlstra <peterz@...radead.org> wrote: > > > >> On Sat, May 11, 2013 at 09:50:08AM +0200, Ingo Molnar wrote: > >> > That's really a red herring: there's absolutely no reason why the > >> > kernel could not pass back the level of precision it provided. > >> > >> All I've been saying is that doing random precision without feedback is > >> confusing. > > > > I agree with that. > > > >> We also don't really have a good feedback channel for this kind of > >> thing. The best I can come up with is tagging each and every sample with > >> the quality it represents. I think we can do with only one extra > >> PERF_RECORD_MISC bit, but it looks like we're quickly running out of > >> those things. > > > > Hm, how about passing precision back to user-space at creation time, in > > the perf_attr data structure? There's no need to pass it back in every > > sample, precision will not really change during the life-time of an event. > > > >> But I think the biggest problem is PEBS's inability do deal with REP > >> prefixes; see this email from Stephane: > >> https://lkml.org/lkml/2011/2/1/177 > >> > >> It is really unfortunate for PEBS to have such a side-effect; but it > >> makes all memset/memcpy/memmove things appear like they have no cost. > >> I'm very sure that will surprise a number of people. > > > > I'd expect PEBS to get gradually better. > > > > Note that at least for user-space, REP MOVS is getting rarer. libc uses > > SSE based memcpy/memset variants - which is not miscounted by PEBS. The > > kernel still uses REP MOVS - but it's a special case because it cannot > > cheaply use vector registers. > > > > The vast majority of code gets measured by cycles:pp more accurately than > > cycles. > > > I don't understand how you come to that conclusion. [...] By frequently looking at cycles:pp output. > [...] I can show you simple examples where this is not true at all (even > without rep mov). That would be useful if there's any practical problem with cycles:pp. In terms of profiling typical kernel and user space functions it does appear to work very well. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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