Messages in this thread | | | Subject | Re: [PATCHES/RFC] Re: A concern about overflow ring buffer mode | From | "Liang, Kan" <> | Date | Mon, 29 Oct 2018 10:33:06 -0400 |
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On 10/29/2018 9:03 AM, Arnaldo Carvalho de Melo wrote: > Em Fri, Oct 26, 2018 at 04:11:51PM -0400, Liang, Kan escreveu: >> On 10/26/2018 3:24 PM, Arnaldo Carvalho de Melo wrote: >>> Em Fri, Oct 26, 2018 at 03:16:29PM -0400, Liang, Kan escreveu: >>>> On 10/26/2018 3:12 PM, Arnaldo Carvalho de Melo wrote: >>>>> Em Fri, Oct 26, 2018 at 03:07:40PM -0400, Liang, Kan escreveu: >>>>>> On 10/26/2018 3:02 PM, Arnaldo Carvalho de Melo wrote: >>>>> I checked and both have the same result. But I still think there is >>>>> value in having the shorter form, ok? > >>>> Sure. > >>> Ok. > >>> I think that we should default back to --no-overwrite till we get this >>> sorted out, as the effect is easily noticeable, as David reported and I >>> reproduced, when doing kernel builds. > >> It is mainly for performance reason to switch to overwrite mode. The impact >> was very small when I did my test. But now the effect is easily noticeable >> in other tests. Yes, I agree. We may change it back to non-overwrite mode >> until the issue is addressed. > > So, I have these two patches in my perf/core branch, with Fixes tags > that will make them get to the stable kernels, ok?
I just realized that the problem in KNL will be back if we switch back to non-overwrite mode. The problem is that users have to wait tens of minutes to see perf top results on the screen in KNL. Before that, there is nothing but a black screen.
Sorry I didn't notice it last Friday. Because I thought the ui_warning in perf_top__mmap_read() can give user a hint. So the user can switch to overwrite mode manually. But unfortunately, the ui_warning doesn't work. Because it is called after perf_top__mmap_read(). The processing time of perf_top__mmap_read() could be tens of minutes.
Thanks, Kan
> > > From f54ef0e7342efb77205b2faaacbcb81cdd31f064 Mon Sep 17 00:00:00 2001 > From: Arnaldo Carvalho de Melo <acme@redhat.com> > Date: Fri, 26 Oct 2018 15:55:23 -0300 > Subject: [PATCH 1/2] perf top: Allow disabling the overwrite mode > > In ebebbf082357 ("perf top: Switch default mode to overwrite mode") we > forgot to leave a way to disable that new default, add a --overwrite > option that can be disabled using --no-overwrite, since the code already > in such a way that we can readily disable this mode. > > This is useful when investigating bugs with this mode like the recent > report from David Miller where lots of unknown symbols appear due to > disabling the events while processing them which disables all record > types, not just PERF_RECORD_SAMPLE, which makes it impossible to resolve > maps when we lose PERF_RECORD_MMAP records. > > This can be easily seen while building a kernel, when there are lots of > short lived processes. > > Reported-by: David Miller <davem@davemloft.net> > Acked-by: Kan Liang <kan.liang@intel.com> > Cc: Adrian Hunter <adrian.hunter@intel.com> > Cc: Andi Kleen <ak@linux.intel.com> > Cc: David Ahern <dsahern@gmail.com> > Cc: Jin Yao <yao.jin@linux.intel.com> > Cc: Jiri Olsa <jolsa@kernel.org> > Cc: Namhyung Kim <namhyung@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Wang Nan <wangnan0@huawei.com> > Fixes: ebebbf082357 ("perf top: Switch default mode to overwrite mode") > Link: https://lkml.kernel.org/n/tip-oqgsz2bq4kgrnnajrafcdhie@git.kernel.org > Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com> > --- > tools/perf/Documentation/perf-top.txt | 5 +++++ > tools/perf/builtin-top.c | 2 ++ > 2 files changed, 7 insertions(+) > > diff --git a/tools/perf/Documentation/perf-top.txt b/tools/perf/Documentation/perf-top.txt > index 114fda12aa49..d4be6061fe1c 100644 > --- a/tools/perf/Documentation/perf-top.txt > +++ b/tools/perf/Documentation/perf-top.txt > @@ -242,6 +242,11 @@ Default is to monitor all CPUS. > --hierarchy:: > Enable hierarchy output. > > +--overwrite:: > + This is the default, but for investigating problems with it or any other strange > + behaviour like lots of unknown samples, we may want to disable this mode by using > + --no-overwrite. > + > --force:: > Don't do ownership validation. > > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c > index d21d8751e749..214fad747b04 100644 > --- a/tools/perf/builtin-top.c > +++ b/tools/perf/builtin-top.c > @@ -1372,6 +1372,8 @@ int cmd_top(int argc, const char **argv) > "Show raw trace event output (do not use print fmt or plugins)"), > OPT_BOOLEAN(0, "hierarchy", &symbol_conf.report_hierarchy, > "Show entries in a hierarchy"), > + OPT_BOOLEAN(0, "overwrite", &top.record_opts.overwrite, > + "Use a backward ring buffer, default: yes"), > OPT_BOOLEAN(0, "force", &symbol_conf.force, "don't complain, do it"), > OPT_UINTEGER(0, "num-thread-synthesize", &top.nr_threads_synthesize, > "number of thread to run event synthesize"), >
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