Re: [PATCH V3 08/14] coresight: core: Add support for dedicated percpu sinks

[Anshuman Khandual]

On 2/5/21 12:04 AM, Mathieu Poirier wrote:
> On Thu, Jan 28, 2021 at 09:16:34AM +0000, Suzuki K Poulose wrote:
>> On 1/27/21 8:55 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@xxxxxxxxxx>
>>> Cc: Mike Leach <mike.leach@xxxxxxxxxx>
>>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>> ---
>>> Changes in V3:
>>>
>>> - Updated coresight_find_default_sink()
>>>
>>>   drivers/hwtracing/coresight/coresight-core.c | 16 ++++++++++++++--
>>>   include/linux/coresight.h                    | 12 ++++++++++++
>>>   2 files changed, 26 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
>>> index 0062c89..4795e28 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);
> 
> The return value for _coresight_build_path() needs to be checked.  Otherwise a
> failure to allocate a node for the sink will go unoticed and make for a very
> hard problem to debug.

How about this instead ?

diff --git a/drivers/hwtracing/coresight/coresight-core.c b/drivers/hwtracing/coresight/coresight-core.c
index 4795e28..e93e669 100644
--- a/drivers/hwtracing/coresight/coresight-core.c
+++ b/drivers/hwtracing/coresight/coresight-core.c
@@ -787,9 +787,10 @@ static int _coresight_build_path(struct coresight_device *csdev,
 
        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;
+               if (_coresight_build_path(sink, sink, path) == 0) {
+                       found = true;
+                       goto out;
+               }
        }
 
        /* Not a sink - recursively explore each port found on this element */

> 
>>> +		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;
>>> @@ -999,8 +1007,12 @@ 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)
>>> -		csdev->def_sink = coresight_find_sink(csdev, &depth);
>>> +	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;
>>>   }
>>> diff --git a/include/linux/coresight.h b/include/linux/coresight.h
>>> index 976ec26..bc3a5ca 100644
>>> --- a/include/linux/coresight.h
>>> +++ b/include/linux/coresight.h
>>> @@ -50,6 +50,7 @@ enum coresight_dev_subtype_sink {
>>>   	CORESIGHT_DEV_SUBTYPE_SINK_PORT,
>>>   	CORESIGHT_DEV_SUBTYPE_SINK_BUFFER,
>>>   	CORESIGHT_DEV_SUBTYPE_SINK_SYSMEM,
>>> +	CORESIGHT_DEV_SUBTYPE_SINK_PERCPU_SYSMEM,
> 
> Do we absolutely need to add a new sink type?  It is only used in
> _coresight_build_path() and that code could be: 
> 
> 	if (coresight_is_percpu_source(csdev)) {
> 	    sink == per_cpu(csdev_sink, source_ops(csdev)->cpu_id(csdev));

Do you mean if (sink == per_cpu(...)) above ?

>             if (sink && sink == csdev) {

How could the sink fetched from the source csdev be the same ?

I would still suggest keeping CORESIGHT_DEV_SUBTYPE_SINK_PERCPU_SYSMEM for
logical separation between source and sink, which also improves clarity
and readability.