From:   Yang Jihong <yangjihong1@huawei.com>
To:     <peterz@infradead.org>, <mingo@redhat.com>, <acme@kernel.org>,
        <mark.rutland@arm.com>, <alexander.shishkin@linux.intel.com>,
        <jolsa@kernel.org>, <namhyung@kernel.org>, <irogers@google.com>,
        <eranian@google.com>, <linux-perf-users@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     <yangjihong1@huawei.com>
Subject: [PATCH RESEND v3] perf/core: Fix hardlockup failure caused by perf throttle
Date:   Mon, 27 Feb 2023 10:35:08 +0800
Message-ID: <20230227023508.102230-1-yangjihong1@huawei.com>
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Precedence: bulk

commit e050e3f0a71bf ("perf: Fix broken interrupt rate throttling")
introduces a change in throttling threshold judgment. Before this,
compare hwc->interrupts and max_samples_per_tick, then increase
hwc->interrupts by 1, but this commit reverses order of these two
behaviors, causing the semantics of max_samples_per_tick to change.
In literal sense of "max_samples_per_tick", if hwc->interrupts ==
max_samples_per_tick, it should not be throttled, therefore, the judgment
condition should be changed to "hwc->interrupts > max_samples_per_tick".

In fact, this may cause the hardlockup to fail, The minimum value of
max_samples_per_tick may be 1, in this case, the return value of
__perf_event_account_interrupt function is 1.
As a result, nmi_watchdog gets throttled, which would stop PMU (Use x86
architecture as an example, see x86_pmu_handle_irq).

Fixes: e050e3f0a71b ("perf: Fix broken interrupt rate throttling")
Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
---

Changes since v2:
  - Add fixed commit.

Changes since v1:
  - Modify commit title.

 kernel/events/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f79fd8b87f75..0540a8653906 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9434,7 +9434,7 @@ __perf_event_account_interrupt(struct perf_event *event, int throttle)
 	} else {
 		hwc->interrupts++;
 		if (unlikely(throttle
-			     && hwc->interrupts >= max_samples_per_tick)) {
+			     && hwc->interrupts > max_samples_per_tick)) {
 			__this_cpu_inc(perf_throttled_count);
 			tick_dep_set_cpu(smp_processor_id(), TICK_DEP_BIT_PERF_EVENTS);
 			hwc->interrupts = MAX_INTERRUPTS;
-- 
2.30.GIT