linux-kernel - [PATCH 5.14 817/849] s390/cpumf: cpum_cf PMU displays invalid value after hotplug remove

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Message-Id: <20211115165447.876809608@linuxfoundation.org>
Date:   Mon, 15 Nov 2021 18:05:00 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Sumanth Korikkar <sumanthk@...ux.ibm.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: [PATCH 5.14 817/849] s390/cpumf: cpum_cf PMU displays invalid value after hotplug remove

From: Thomas Richter <tmricht@...ux.ibm.com>

commit 9d48c7afedf91a02d03295837ec76b2fb5e7d3fe upstream.

When a CPU is hotplugged while the perf stat -e cycles command is
running, a wrong (very large) value is displayed immediately after the
CPU removal:

  Check the values, shouldn't be too high as in
            time             counts unit events
     1.001101919           29261846      cycles
     2.002454499           17523405      cycles
     3.003659292           24361161      cycles
     4.004816983 18446744073638406144      cycles
     5.005671647      <not counted>      cycles
     ...

The CPU hotplug off took place after 3 seconds.
The issue is the read of the event count value after 4 seconds when
the CPU is not available and the read of the counter returns an
error. This is treated as a counter value of zero. This results
in a very large value (0 - previous_value).

Fix this by detecting the hotplugged off CPU and report 0 instead
of a very large number.

Cc: stable@...r.kernel.org
Fixes: a029a4eab39e ("s390/cpumf: Allow concurrent access for CPU Measurement Counter Facility")
Reported-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
Signed-off-by: Thomas Richter <tmricht@...ux.ibm.com>
Reviewed-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/s390/kernel/perf_cpum_cf.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/s390/kernel/perf_cpum_cf.c
+++ b/arch/s390/kernel/perf_cpum_cf.c
@@ -679,8 +679,10 @@ static void cpumf_pmu_stop(struct perf_e
 						      false);
 			if (cfdiag_diffctr(cpuhw, event->hw.config_base))
 				cfdiag_push_sample(event, cpuhw);
-		} else
+		} else if (cpuhw->flags & PMU_F_RESERVED) {
+			/* Only update when PMU not hotplugged off */
 			hw_perf_event_update(event);
+		}
 		hwc->state |= PERF_HES_UPTODATE;
 	}
 }