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    Subject[PATCH 4.4 10/59] s390/cpum_sf: Avoid SBD overflow condition in irq handler
    Date
    From: Thomas Richter <tmricht@linux.ibm.com>

    [ Upstream commit 0539ad0b22877225095d8adef0c376f52cc23834 ]

    The s390 CPU Measurement sampling facility has an overflow condition
    which fires when all entries in a SBD are used.
    The measurement alert interrupt is triggered and reads out all samples
    in this SDB. It then tests the successor SDB, if this SBD is not full,
    the interrupt handler does not read any samples at all from this SDB
    The design waits for the hardware to fill this SBD and then trigger
    another meassurement alert interrupt.

    This scheme works nicely until
    an perf_event_overflow() function call discards the sample due to
    a too high sampling rate.
    The interrupt handler has logic to read out a partially filled SDB
    when the perf event overflow condition in linux common code is met.
    This causes the CPUM sampling measurement hardware and the PMU
    device driver to operate on the same SBD's trailer entry.
    This should not happen.

    This can be seen here using this trace:
    cpumsf_pmu_add: tear:0xb5286000
    hw_perf_event_update: sdbt 0xb5286000 full 1 over 0 flush_all:0
    hw_perf_event_update: sdbt 0xb5286008 full 0 over 0 flush_all:0
    above shows 1. interrupt
    hw_perf_event_update: sdbt 0xb5286008 full 1 over 0 flush_all:0
    hw_perf_event_update: sdbt 0xb5286008 full 0 over 0 flush_all:0
    above shows 2. interrupt
    ... this goes on fine until...
    hw_perf_event_update: sdbt 0xb5286068 full 1 over 0 flush_all:0
    perf_push_sample1: overflow
    one or more samples read from the IRQ handler are rejected by
    perf_event_overflow() and the IRQ handler advances to the next SDB
    and modifies the trailer entry of a partially filled SDB.
    hw_perf_event_update: sdbt 0xb5286070 full 0 over 0 flush_all:1
    timestamp: 14:32:52.519953

    Next time the IRQ handler is called for this SDB the trailer entry shows
    an overflow count of 19 missed entries.
    hw_perf_event_update: sdbt 0xb5286070 full 1 over 19 flush_all:1
    timestamp: 14:32:52.970058

    Remove access to a follow on SDB when event overflow happened.

    Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
    Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/s390/kernel/perf_cpum_sf.c | 6 ------
    1 file changed, 6 deletions(-)

    diff --git a/arch/s390/kernel/perf_cpum_sf.c b/arch/s390/kernel/perf_cpum_sf.c
    index 01766671fa2a..4a76b381d25a 100644
    --- a/arch/s390/kernel/perf_cpum_sf.c
    +++ b/arch/s390/kernel/perf_cpum_sf.c
    @@ -1294,12 +1294,6 @@ static void hw_perf_event_update(struct perf_event *event, int flush_all)
    */
    if (flush_all && done)
    break;
    -
    - /* If an event overflow happened, discard samples by
    - * processing any remaining sample-data-blocks.
    - */
    - if (event_overflow)
    - flush_all = 1;
    }

    /* Account sample overflows in the event hardware structure */
    --
    2.20.1


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