linux-kernel - Re: [PATCH v2] coresight: tmc-etr: Fix perf

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Date:   Wed, 14 Aug 2019 17:00:30 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Yabin Cui <yabinc@...gle.com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] coresight: tmc-etr: Fix perf_data check.

Hi Yabin,

On Mon, 12 Aug 2019 at 13:03, Yabin Cui <yabinc@...gle.com> wrote:
>
> When tracing etm data of multiple threads on multiple cpus through
> perf interface, each cpu has a unique etr_perf_buffer while sharing
> the same etr device. There is no guarantee that the last cpu starts
> etm tracing also stops last. This makes perf_data check fail.

Did you actually see the check fail or is this a theoretical thing?
I'm really perplex here has I have tested this scenario many times
without issues.

>
> Fix it by checking etr_buf instead of etr_perf_buffer.
>
> Fixes: 3147da92a8a8 ("coresight: tmc-etr: Allocate and free ETR memory buffers for CPU-wide scenarios")
> Signed-off-by: Yabin Cui <yabinc@...gle.com>
> ---
>
> v1 -> v2: rename perf_data to perf_buf. Add fixes tag.
>
> ---
>  drivers/hwtracing/coresight/coresight-tmc-etr.c | 6 +++---
>  drivers/hwtracing/coresight/coresight-tmc.h     | 6 +++---
>  2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/hwtracing/coresight/coresight-tmc-etr.c b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> index 17006705287a..90d1548ad268 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc-etr.c
> +++ b/drivers/hwtracing/coresight/coresight-tmc-etr.c
> @@ -1484,7 +1484,7 @@ tmc_update_etr_buffer(struct coresight_device *csdev,
>                 goto out;
>         }
>
> -       if (WARN_ON(drvdata->perf_data != etr_perf)) {
> +       if (WARN_ON(drvdata->perf_buf != etr_buf)) {

In CPU wide scenarios each perf event (one per CPU) is associated with
an event_data during the setup process.  The event_data is the
etr_perf holding a reference to the perf ring buffer for that specific
event along with the etr_buf, regardless of who created the latter.
>From there, when the event is installed on a CPU, the csdev for that
CPU is given a reference to the event_data of that event[1].  Before
going further notice how there is a per CPU csdev and event handle to
keep track of event specifics[2]. As such both (per CPU) csdev and
event handle carry the exact same reference to the etr_perf.

When an event is stopped the exact same per CPU references are
taken[3] and later fed to sink->update_buffer().  If
sink->update_buffer() is called on an event (again one per CPU)
function etm_event_start() must have been called and
(drvdata->perf_data == etf_perf) needs to hold.

If that is not the case, an event from a completely different trace
session has been installed on that CPU and that can't happen thanks to
drvdata->id.

As such if you get a condition where (drvdata->perf_buf != etr_buf),
one of two things has happened:

1) Something went seriously wrong and the WARN_ON() did its job.
2) A corner case is manifesting in your test environment that I
haven't provisioned for.  If that is the case we need to figure out
exactly what is happening before considering a fix.

Thanks,
Mathieu

[1]. https://elixir.bootlin.com/linux/v5.3-rc4/source/drivers/hwtracing/coresight/coresight-tmc-etr.c#L1559
[2]. https://elixir.bootlin.com/linux/v5.3-rc4/source/drivers/hwtracing/coresight/coresight-etm-perf.c#L298
[3]. https://elixir.bootlin.com/linux/v5.3-rc4/source/drivers/hwtracing/coresight/coresight-etm-perf.c#L348

>                 lost = true;
>                 spin_unlock_irqrestore(&drvdata->spinlock, flags);
>                 goto out;
> @@ -1497,7 +1497,7 @@ tmc_update_etr_buffer(struct coresight_device *csdev,
>
>         CS_LOCK(drvdata->base);
>         /* Reset perf specific data */
> -       drvdata->perf_data = NULL;
> +       drvdata->perf_buf = NULL;
>         spin_unlock_irqrestore(&drvdata->spinlock, flags);
>
>         size = etr_buf->len;
> @@ -1556,7 +1556,7 @@ static int tmc_enable_etr_sink_perf(struct coresight_device *csdev, void *data)
>         }
>
>         etr_perf->head = PERF_IDX2OFF(handle->head, etr_perf);
> -       drvdata->perf_data = etr_perf;
> +       drvdata->perf_buf = etr_perf->etr_buf;
>
>         /*
>          * No HW configuration is needed if the sink is already in
> diff --git a/drivers/hwtracing/coresight/coresight-tmc.h b/drivers/hwtracing/coresight/coresight-tmc.h
> index 1ed50411cc3c..f9a0c95e9ba2 100644
> --- a/drivers/hwtracing/coresight/coresight-tmc.h
> +++ b/drivers/hwtracing/coresight/coresight-tmc.h
> @@ -178,8 +178,8 @@ struct etr_buf {
>   *             device configuration register (DEVID)
>   * @idr:       Holds etr_bufs allocated for this ETR.
>   * @idr_mutex: Access serialisation for idr.
> - * @perf_data: PERF buffer for ETR.
> - * @sysfs_data:        SYSFS buffer for ETR.
> + * @sysfs_buf: SYSFS buffer for ETR.
> + * @perf_buf:  PERF buffer for ETR.
>   */
>  struct tmc_drvdata {
>         void __iomem            *base;
> @@ -202,7 +202,7 @@ struct tmc_drvdata {
>         struct idr              idr;
>         struct mutex            idr_mutex;
>         struct etr_buf          *sysfs_buf;
> -       void                    *perf_data;
> +       struct etr_buf          *perf_buf;
>  };
>
>  struct etr_buf_operations {
> --
> 2.23.0.rc1.153.gdeed80330f-goog
>