From: Peter Zijlstra <peterz@infradead.org> To: Andi Kleen <ak@linux.intel.com> Cc: linux-kernel@vger.kernel.org, Vince Weaver <vince@deater.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, Stephane Eranian <eranian@google.com>, Alexander Shishkin <alexander.shishkin@linux.intel.com>, Arnaldo Carvalho de Melo <acme@kernel.org>, Jiri Olsa <jolsa@kernel.org> Subject: Re: perf group read for inherited events Date: Tue, 30 May 2017 11:45:12 +0200 Message-ID: <20170530094512.dy2nljns2uq7qa3j@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <20170526205601.GA5271@tassilo.jf.intel.com> On Fri, May 26, 2017 at 01:56:01PM -0700, Andi Kleen wrote: > I have a need for read group reads for inherited events. > > It looks like the perf group read code already has all the code > to handle inheritance, __perf_read_group_add walks > the children list and adds them all up. > > 4409 > 4410 list_for_each_entry(sub, &leader->sibling_list, group_entry) { > 4411 values[n++] += perf_event_count(sub); > 4412 if (read_format & PERF_FORMAT_ID) > 4413 values[n++] = primary_event_id(sub); > 4414 } > > > I disabled the check that forbids this and it seems to work > from some simple testing. > > Again do I miss something why this was disabled? The thing that seems difficult is PERF_SAMPLE_READ vs inherited, irrespective of PERF_FORMAT_GROUP. The error seems to be in that patch you fingered: 3dab77fb1bf8 ("perf: Rework/fix the whole read vs group stuff") - PERF_SAMPLE_GROUP = 1U << 4, + PERF_SAMPLE_READ = 1U << 4, - if (attr->inherit && (attr->sample_type & PERF_SAMPLE_GROUP)) + if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP)) is a clear fail :/ > Perhaps some locking issue? There are some comments on it, > but I'm not sure I understand all the subtleties: > > /* > * By locking the child_mutex of the leader we effectively > * lock the child list of all siblings.. XXX explain how. > */ > mutex_lock(&leader->child_mutex); This is more recent. Here I failed to find a coherent text to explain the locking. It is correct through. I think its something like: @@ -4426,8 +4426,9 @@ static int perf_read_group(struct perf_event *event, values[0] = 1 + leader->nr_siblings; /* - * By locking the child_mutex of the leader we effectively - * lock the child list of all siblings.. XXX explain how. + * By locking the child_mutex of the leader we effectively lock the + * child list of all siblings. Since inherit_group() will first clone + * the leader and will this be blocked on us holding its child_mutex. */ mutex_lock(&leader->child_mutex); > Or is the simple patch below good enough? The below seems to be the correct thing. It is rather unfortunate that this would break/significantly change existing semantics :/ --- kernel/events/core.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index 8d6acaeeea17..2d9de6fb9a5a 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5722,9 +5722,6 @@ static void perf_output_read_one(struct perf_output_handle *handle, __output_copy(handle, values, n * sizeof(u64)); } -/* - * XXX PERF_FORMAT_GROUP vs inherited events seems difficult. - */ static void perf_output_read_group(struct perf_output_handle *handle, struct perf_event *event, u64 enabled, u64 running) @@ -5769,6 +5766,12 @@ static void perf_output_read_group(struct perf_output_handle *handle, #define PERF_FORMAT_TOTAL_TIMES (PERF_FORMAT_TOTAL_TIME_ENABLED|\ PERF_FORMAT_TOTAL_TIME_RUNNING) +/* + * XXX PERF_SAMPLE_READ vs inherited events seems difficult. + * + * The problem is that its both hard and excessively expensive to iterate the + * child list from interrupt/NMI context. + */ static void perf_output_read(struct perf_output_handle *handle, struct perf_event *event) { @@ -9434,9 +9437,10 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu, local64_set(&hwc->period_left, hwc->sample_period); /* - * we currently do not support PERF_FORMAT_GROUP on inherited events + * We currently do not support PERF_SAMPLE_READ on inherited events. + * See perf_output_read(). */ - if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP)) + if (attr->inherit && (attr->sample_type & PERF_SAMPLE_READ)) goto err_ns; if (!has_branch_stack(event))
next parent reply index Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <20170526205601.GA5271@tassilo.jf.intel.com> 2017-05-30 9:45 ` Peter Zijlstra [this message] 2017-05-30 13:57 ` Andi Kleen 2017-05-30 17:01 ` Peter Zijlstra 2017-05-30 17:55 ` Vince Weaver 2017-05-30 18:53 ` Peter Zijlstra 2017-05-31 21:08 ` Vince Weaver 2017-06-08 9:21 ` [tip:perf/core] perf/core: Correct event creation with PERF_FORMAT_GROUP tip-bot for Peter Zijlstra
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