[PATCH 5.9 094/757] perf/x86/intel/ds: Fix x86_pmu_stop warning for large PEBS

Greg Kroah-Hartman
2020-10-27T15:23:51Z

From: Kan Liang <kan.liang@linux.intel.com>

[ Upstream commit 35d1ce6bec133679ff16325d335217f108b84871 ]

A warning as below may be triggered when sampling with large PEBS.

[ 410.411250] perf: interrupt took too long (72145 > 71975), lowering
kernel.perf_event_max_sample_rate to 2000
[ 410.724923] ------------[ cut here ]------------
[ 410.729822] WARNING: CPU: 0 PID: 16397 at arch/x86/events/core.c:1422
x86_pmu_stop+0x95/0xa0
[ 410.933811] x86_pmu_del+0x50/0x150
[ 410.937304] event_sched_out.isra.0+0xbc/0x210
[ 410.941751] group_sched_out.part.0+0x53/0xd0
[ 410.946111] ctx_sched_out+0x193/0x270
[ 410.949862] __perf_event_task_sched_out+0x32c/0x890
[ 410.954827] ? set_next_entity+0x98/0x2d0
[ 410.958841] __schedule+0x592/0x9c0
[ 410.962332] schedule+0x5f/0xd0
[ 410.965477] exit_to_usermode_loop+0x73/0x120
[ 410.969837] prepare_exit_to_usermode+0xcd/0xf0
[ 410.974369] ret_from_intr+0x2a/0x3a
[ 410.977946] RIP: 0033:0x40123c
[ 411.079661] ---[ end trace bc83adaea7bb664a ]---

In the non-overflow context, e.g., context switch, with large PEBS, perf
may stop an event twice. An example is below.

//max_samples_per_tick is adjusted to 2
//NMI is triggered
intel_pmu_handle_irq()
handle_pmi_common()
drain_pebs()
__intel_pmu_pebs_event()
perf_event_overflow()
__perf_event_account_interrupt()
hwc->interrupts = 1
return 0
//A context switch happens right after the NMI.
//In the same tick, the perf_throttled_seq is not changed.
perf_event_task_sched_out()
perf_pmu_sched_task()
intel_pmu_drain_pebs_buffer()
__intel_pmu_pebs_event()
perf_event_overflow()
__perf_event_account_interrupt()
++hwc->interrupts >= max_samples_per_tick
return 1
x86_pmu_stop(); # First stop
perf_event_context_sched_out()
task_ctx_sched_out()
ctx_sched_out()
event_sched_out()
x86_pmu_del()
x86_pmu_stop(); # Second stop and trigger the warning

Perf should only invoke the perf_event_overflow() in the overflow
context.

Current drain_pebs() is called from:
- handle_pmi_common() -- overflow context
- intel_pmu_pebs_sched_task() -- non-overflow context
- intel_pmu_pebs_disable() -- non-overflow context
- intel_pmu_auto_reload_read() -- possible overflow context
With PERF_SAMPLE_READ + PERF_FORMAT_GROUP, the function may be
invoked in the NMI handler. But, before calling the function, the
PEBS buffer has already been drained. The __intel_pmu_pebs_event()
will not be called in the possible overflow context.

To fix the issue, an indicator is required to distinguish between the
overflow context aka handle_pmi_common() and other cases.
The dummy regs pointer can be used as the indicator.

In the non-overflow context, perf should treat the last record the same
as other PEBS records, and doesn't invoke the generic overflow handler.

Fixes: 21509084f999 ("perf/x86/intel: Handle multiple records in the PEBS buffer")
Reported-by: Like Xu <like.xu@linux.intel.com>
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Like Xu <like.xu@linux.intel.com>
Link: https://lkml.kernel.org/r/20200902210649.2743-1-kan.liang@linux.intel.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/events/intel/ds.c | 32 ++++++++++++++++++++------------
1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index 86848c57b55ed..404315df1e167 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -670,9 +670,7 @@ int intel_pmu_drain_bts_buffer(void)

static inline void intel_pmu_drain_pebs_buffer(void)
{
- struct pt_regs regs;
-
- x86_pmu.drain_pebs(&regs);
+ x86_pmu.drain_pebs(NULL);
}

/*
@@ -1737,6 +1735,7 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
struct x86_perf_regs perf_regs;
struct pt_regs *regs = &perf_regs.regs;
void *at = get_next_pebs_record_by_bit(base, top, bit);
+ struct pt_regs dummy_iregs;

if (hwc->flags & PERF_X86_EVENT_AUTO_RELOAD) {
/*
@@ -1749,6 +1748,9 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
} else if (!intel_pmu_save_and_restart(event))
return;

+ if (!iregs)
+ iregs = &dummy_iregs;
+
while (count > 1) {
setup_sample(event, iregs, at, &data, regs);
perf_event_output(event, &data, regs);
@@ -1758,16 +1760,22 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
}

setup_sample(event, iregs, at, &data, regs);
-
- /*
- * All but the last records are processed.
- * The last one is left to be able to call the overflow handler.
- */
- if (perf_event_overflow(event, &data, regs)) {
- x86_pmu_stop(event, 0);
- return;
+ if (iregs == &dummy_iregs) {
+ /*
+ * The PEBS records may be drained in the non-overflow context,
+ * e.g., large PEBS + context switch. Perf should treat the
+ * last record the same as other PEBS records, and doesn't
+ * invoke the generic overflow handler.
+ */
+ perf_event_output(event, &data, regs);
+ } else {
+ /*
+ * All but the last records are processed.
+ * The last one is left to be able to call the overflow handler.
+ */
+ if (perf_event_overflow(event, &data, regs))
+ x86_pmu_stop(event, 0);
}
-
}

static void intel_pmu_drain_pebs_core(struct pt_regs *iregs)
--
2.25.1