linux-kernel - Re: [PATCH V2 08/11] coresight: core: Add support for dedicated percpu sinks

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Message-ID: <1ae508a9-69f5-af33-acd0-d305f6039187@arm.com>
Date:   Fri, 15 Jan 2021 12:31:14 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org
Cc:     mathieu.poirier@...aro.org, mike.leach@...aro.org,
        Linu Cherian <lcherian@...vell.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 08/11] coresight: core: Add support for dedicated
 percpu sinks

On 1/15/21 2:36 AM, Anshuman Khandual wrote:
> 
> 
> On 1/13/21 3:13 PM, Suzuki K Poulose wrote:
>> On 1/13/21 4:18 AM, Anshuman Khandual wrote:
>>> Add support for dedicated sinks that are bound to individual CPUs. (e.g,
>>> TRBE). To allow quicker access to the sink for a given CPU bound source,
>>> keep a percpu array of the sink devices. Also, add support for building
>>> a path to the CPU local sink from the ETM.
>>>
>>> This adds a new percpu sink type CORESIGHT_DEV_SUBTYPE_SINK_PERCPU_SYSMEM.
>>> This new sink type is exclusively available and can only work with percpu
>>> source type device CORESIGHT_DEV_SUBTYPE_SOURCE_PERCPU_PROC.
>>>
>>> This defines a percpu structure that accommodates a single coresight_device
>>> which can be used to store an initialized instance from a sink driver. As
>>> these sinks are exclusively linked and dependent on corresponding percpu
>>> sources devices, they should also be the default sink device during a perf
>>> session.
>>>
>>> Outwards device connections are scanned while establishing paths between a
>>> source and a sink device. But such connections are not present for certain
>>> percpu source and sink devices which are exclusively linked and dependent.
>>> Build the path directly and skip connection scanning for such devices.
>>>
>>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>>> Cc: Mike Leach <mike.leach@...aro.org>
>>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>> ---
>>>    drivers/hwtracing/coresight/coresight-core.c | 14 ++++++++++++++
>>>    include/linux/coresight.h                    | 12 ++++++++++++
>>>    2 files changed, 26 insertions(+)
>>>
>>> diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
>>> index 0062c89..b300606 100644
>>> --- a/drivers/hwtracing/coresight/coresight-core.c
>>> +++ b/drivers/hwtracing/coresight/coresight-core.c
>>> @@ -23,6 +23,7 @@
>>>    #include "coresight-priv.h"
>>>      static DEFINE_MUTEX(coresight_mutex);
>>> +DEFINE_PER_CPU(struct coresight_device *, csdev_sink);
>>>      /**
>>>     * struct coresight_node - elements of a path, from source to sink
>>> @@ -784,6 +785,13 @@ static int _coresight_build_path(struct coresight_device *csdev,
>>>        if (csdev == sink)
>>>            goto out;
>>>    +    if (coresight_is_percpu_source(csdev) && coresight_is_percpu_sink(sink) &&
>>> +        sink == per_cpu(csdev_sink, source_ops(csdev)->cpu_id(csdev))) {
>>> +        _coresight_build_path(sink, sink, path);
>>> +        found = true;
>>> +        goto out;
>>> +    }
>>> +
>>>        /* Not a sink - recursively explore each port found on this element */
>>>        for (i = 0; i < csdev->pdata->nr_outport; i++) {
>>>            struct coresight_device *child_dev;
>>> @@ -998,6 +1006,12 @@ coresight_find_default_sink(struct coresight_device *csdev)
>>>    {
>>>        int depth = 0;
>>>    +    if (coresight_is_percpu_source(csdev)) {
>>
>> On a system without per_cpu sink, this would reset the default sink for the source device
>> every single time and fallback to searching every single time.
> 
> Right.
> 
>> So I think it would be better if did check if the def_sink was not set.
>> We could fold this into the case below may be. i.e,
>>
>>
>>      if (!csdev->def_sink) {
>>          if (coresight_is_percpu_source(csdev))
>>              csdev->def_sink = per_cpu(csdev_sink, source_ops(csdev)->cpu_id(csdev));
>>          if (!csdev->def_sink)            csdev->def_sink = coresight_find_sink(csdev, &depth);
>>      }
>>
>> Otherwise looks good to me.
> 
> struct coresight_device *
> coresight_find_default_sink(struct coresight_device *csdev)
> {
>          int depth = 0;
> 
>          /* look for a default sink if we have not found for this device */
>          if (!csdev->def_sink) {
>                  if (coresight_is_percpu_source(csdev))
>                          csdev->def_sink = per_cpu(csdev_sink, source_ops(csdev)->cpu_id(csdev));
>                  if (!csdev->def_sink)
>                          csdev->def_sink = coresight_find_sink(csdev, &depth);
>          }
>          return csdev->def_sink;
> }
> 
> Would this be better instead ? coresight_find_sink() is invoked both when the
> source is not percpu (traditional coresight sources) and also as a fallback in
> case a percpu sink is not found for the percpu source device.
Yes, this is exactly what I proposed above.

Cheers
Suzuki