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Date: Wed, 26 Feb 2014 00:59:10 -0500 (EST)
From: Vince Weaver <vincent.weaver@...ne.edu>
To: Peter Zijlstra <peterz@...radead.org>
cc: Vince Weaver <vincent.weaver@...ne.edu>,
Dave Jones <davej@...hat.com>,
Linux Kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...hat.com>,
Paul Mackerras <paulus@...ba.org>,
Steven Rostedt <rostedt@...dmis.org>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: x86_pmu_start WARN_ON.
On Mon, 24 Feb 2014, Peter Zijlstra wrote:
> On Fri, Feb 21, 2014 at 03:18:38PM -0500, Vince Weaver wrote:
> > I've applied the patch and have been unable to trigger the warning with
> > either my testcase or a few hours of fuzzing.
>
> Yay.
>
> > My only comment on the patch is it could always use some comments.
> >
> > The perf_event code is really hard to follow as is, without adding
> > more uncommented special cases.
>
> Does the below help a bit? Or is there anywhere in particular you want
> more comments?
yes, every little bit helps.
While chasing these fuzzer-related bugs I end up deep in the perf_event
code and many of the routines have no comments at all. Eventually I have
to dig out the K+R book to figure out order precendece of ++ prefix
operators, have at least 2-3 different files open in editors, plus a bunch
of firefox tabs open to http://lxr.free-electrons.com, and even then I
misunderstand the code a lot.
Vince
>
> ---
> Subject: perf, x86: Add a few more comments
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Mon Feb 24 12:26:21 CET 2014
>
> Add a few comments on the ->add(), ->del() and ->*_txn()
> implementation.
>
> Requested-by: Vince Weaver <vincent.weaver@...ne.edu>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
> arch/x86/kernel/cpu/perf_event.c | 49 +++++++++++++++++++++++++++------------
> arch/x86/kernel/cpu/perf_event.h | 8 +++---
> 2 files changed, 39 insertions(+), 18 deletions(-)
>
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -892,7 +892,6 @@ static void x86_pmu_enable(struct pmu *p
> * hw_perf_group_sched_in() or x86_pmu_enable()
> *
> * step1: save events moving to new counters
> - * step2: reprogram moved events into new counters
> */
> for (i = 0; i < n_running; i++) {
> event = cpuc->event_list[i];
> @@ -918,6 +917,9 @@ static void x86_pmu_enable(struct pmu *p
> x86_pmu_stop(event, PERF_EF_UPDATE);
> }
>
> + /*
> + * step2: reprogram moved events into new counters
> + */
> for (i = 0; i < cpuc->n_events; i++) {
> event = cpuc->event_list[i];
> hwc = &event->hw;
> @@ -1043,7 +1045,7 @@ static int x86_pmu_add(struct perf_event
> /*
> * If group events scheduling transaction was started,
> * skip the schedulability test here, it will be performed
> - * at commit time (->commit_txn) as a whole
> + * at commit time (->commit_txn) as a whole.
> */
> if (cpuc->group_flag & PERF_EVENT_TXN)
> goto done_collect;
> @@ -1058,6 +1060,10 @@ static int x86_pmu_add(struct perf_event
> memcpy(cpuc->assign, assign, n*sizeof(int));
>
> done_collect:
> + /*
> + * Commit the collect_events() state. See x86_pmu_del() and
> + * x86_pmu_*_txn().
> + */
> cpuc->n_events = n;
> cpuc->n_added += n - n0;
> cpuc->n_txn += n - n0;
> @@ -1183,28 +1189,38 @@ static void x86_pmu_del(struct perf_even
> * If we're called during a txn, we don't need to do anything.
> * The events never got scheduled and ->cancel_txn will truncate
> * the event_list.
> + *
> + * XXX assumes any ->del() called during a TXN will only be on
> + * an event added during that same TXN.
> */
> if (cpuc->group_flag & PERF_EVENT_TXN)
> return;
>
> + /*
> + * Not a TXN, therefore cleanup properly.
> + */
> x86_pmu_stop(event, PERF_EF_UPDATE);
>
> for (i = 0; i < cpuc->n_events; i++) {
> - if (event == cpuc->event_list[i]) {
> -
> - if (i >= cpuc->n_events - cpuc->n_added)
> - --cpuc->n_added;
> + if (event == cpuc->event_list[i])
> + break;
> + }
>
> - if (x86_pmu.put_event_constraints)
> - x86_pmu.put_event_constraints(cpuc, event);
> + if (WARN_ON_ONCE(i == cpuc->n_events)) /* called ->del() without ->add() ? */
> + return;
>
> - while (++i < cpuc->n_events)
> - cpuc->event_list[i-1] = cpuc->event_list[i];
> + /* If we have a newly added event; make sure to decrease n_added. */
> + if (i >= cpuc->n_events - cpuc->n_added)
> + --cpuc->n_added;
> +
> + if (x86_pmu.put_event_constraints)
> + x86_pmu.put_event_constraints(cpuc, event);
> +
> + /* Delete the array entry. */
> + while (++i < cpuc->n_events)
> + cpuc->event_list[i-1] = cpuc->event_list[i];
> + --cpuc->n_events;
>
> - --cpuc->n_events;
> - break;
> - }
> - }
> perf_event_update_userpage(event);
> }
>
> @@ -1598,7 +1614,8 @@ static void x86_pmu_cancel_txn(struct pm
> {
> __this_cpu_and(cpu_hw_events.group_flag, ~PERF_EVENT_TXN);
> /*
> - * Truncate the collected events.
> + * Truncate collected array by the number of events added in this
> + * transaction. See x86_pmu_add() and x86_pmu_*_txn().
> */
> __this_cpu_sub(cpu_hw_events.n_added, __this_cpu_read(cpu_hw_events.n_txn));
> __this_cpu_sub(cpu_hw_events.n_events, __this_cpu_read(cpu_hw_events.n_txn));
> @@ -1609,6 +1626,8 @@ static void x86_pmu_cancel_txn(struct pm
> * Commit group events scheduling transaction
> * Perform the group schedulability test as a whole
> * Return 0 if success
> + *
> + * Does not cancel the transaction on failure; expects the caller to do this.
> */
> static int x86_pmu_commit_txn(struct pmu *pmu)
> {
> --- a/arch/x86/kernel/cpu/perf_event.h
> +++ b/arch/x86/kernel/cpu/perf_event.h
> @@ -130,9 +130,11 @@ struct cpu_hw_events {
> unsigned long running[BITS_TO_LONGS(X86_PMC_IDX_MAX)];
> int enabled;
>
> - int n_events;
> - int n_added;
> - int n_txn;
> + int n_events; /* the # of events in the below arrays */
> + int n_added; /* the # last events in the below arrays;
> + they've never been enabled yet */
> + int n_txn; /* the # last events in the below arrays;
> + added in the current transaction */
> int assign[X86_PMC_IDX_MAX]; /* event to counter assignment */
> u64 tags[X86_PMC_IDX_MAX];
> struct perf_event *event_list[X86_PMC_IDX_MAX]; /* in enabled order */
>
--
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