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

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Message-ID: <20210216204419.GB2936551@xps15>
Date:   Tue, 16 Feb 2021 13:44:19 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org,
        mike.leach@...aro.org, lcherian@...vell.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V3 08/14] coresight: core: Add support for dedicated
 percpu sinks

On Tue, Feb 16, 2021 at 04:10:18PM +0530, Anshuman Khandual wrote:
> 
> 
> 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@...aro.org>
> >>> Cc: Mike Leach <mike.leach@...aro.org>
> >>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> >>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> >>> ---
> >>> 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;
> +               }

I am missing the context now but it is a step in the right direction.  I will
re-assess on your next revision.

>         }
>  
>         /* 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 ?

The above should have been:

        if (coresight_is_percpu_source(csdev)) {
                per_cpu_sink == per_cpu(csdev_sink, source_ops(csdev)->cpu_id(csdev));
                if (per_cpu_sink && per_cpu_sink == sink) {

Apologies for the confusion.

Mathieu

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