Re: [PATCH] coresight: etm: Make cycle count threshold user configurable

[James Clark]

On 04/08/2023 11:10, Al Grant wrote:
> 
> 
>> -----Original Message-----
>> From: James Clark <james.clark@xxxxxxx>
>> Sent: Friday, August 4, 2023 10:23 AM
>> To: Anshuman Khandual <Anshuman.Khandual@xxxxxxx>; Al Grant
>> <Al.Grant@xxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>> Cc: Mike Leach <mike.leach@xxxxxxxxxx>; coresight@xxxxxxxxxxxxxxxx; linux-
>> doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>> Subject: Re: [PATCH] coresight: etm: Make cycle count threshold user
>> configurable
>>
>>
>>
>> On 04/08/2023 09:45, Anshuman Khandual wrote:
>>>
>>>
>>> On 8/4/23 13:34, Al Grant wrote:
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>>>> Sent: Friday, August 4, 2023 5:47 AM
>>>>> To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>>> Cc: Anshuman Khandual <Anshuman.Khandual@xxxxxxx>; Mike Leach
>>>>> <mike.leach@xxxxxxxxxx>; coresight@xxxxxxxxxxxxxxxx;
>>>>> linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>>>>> Subject: [PATCH] coresight: etm: Make cycle count threshold user
>>>>> configurable
>>>>>
>>>>> Cycle counting is enabled, when requested and supported but with a
>>>>> default threshold value ETM_CYC_THRESHOLD_DEFAULT i.e 0x100 getting
>>>>> into TRCCCCTLR, representing the minimum interval between cycle
>>>>> count trace packets.
>>>>>
>>>>> This makes cycle threshold user configurable, from the user space
>>>>> via perf event attributes. Although it falls back using
>>>>> ETM_CYC_THRESHOLD_DEFAULT, in case no explicit request. As expected it
>> creates a sysfs file as well.
>>>>>
>>>>> /sys/bus/event_source/devices/cs_etm/format/cc_threshold
>>>>>
>>>>> New 'cc_threshold' uses 'event->attr.config3' as no more space is
>>>>> available in 'event->attr.config1' or 'event->attr.config2'.
>>>>>
>>>>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>>>> Cc: Mike Leach <mike.leach@xxxxxxxxxx>
>>>>> Cc: James Clark <james.clark@xxxxxxx>
>>>>> Cc: Leo Yan <leo.yan@xxxxxxxxxx>
>>>>> Cc: coresight@xxxxxxxxxxxxxxxx
>>>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>>> Cc: linux-doc@xxxxxxxxxxxxxxx
>>>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>>>> ---
>>>>>  Documentation/trace/coresight/coresight.rst        |  2 ++
>>>>>  drivers/hwtracing/coresight/coresight-etm-perf.c   |  2 ++
>>>>>  drivers/hwtracing/coresight/coresight-etm4x-core.c | 12
>>>>> ++++++++++--
>>>>>  3 files changed, 14 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/trace/coresight/coresight.rst
>>>>> b/Documentation/trace/coresight/coresight.rst
>>>>> index 4a71ea6cb390..b88d83b59531 100644
>>>>> --- a/Documentation/trace/coresight/coresight.rst
>>>>> +++ b/Documentation/trace/coresight/coresight.rst
>>>>> @@ -624,6 +624,8 @@ They are also listed in the folder
>>>>> /sys/bus/event_source/devices/cs_etm/format/
>>>>>     * - timestamp
>>>>>       - Session local version of the system wide setting:
>>>>> :ref:`ETMv4_MODE_TIMESTAMP
>>>>>         <coresight-timestamp>`
>>>>> +   * - cc_treshold
>>>>
>>>> Spelling: cc_threshold
>>>
>>> Will fix this, besides does it require some more description for this
>>> new config option i.e cc_threshold ?
>>>
>>>>
>>>>> +     - Cycle count treshhold value
>>>>>
>>>>>  How to use the STM module
>>>>>  -------------------------
>>>>> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c
>>>>> b/drivers/hwtracing/coresight/coresight-etm-perf.c
>>>>> index 5ca6278baff4..09f75dffae60 100644
>>>>> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
>>>>> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
>>>>> @@ -68,6 +68,7 @@ PMU_FORMAT_ATTR(preset,		"config:0-3");
>>>>>  PMU_FORMAT_ATTR(sinkid,		"config2:0-31");
>>>>>  /* config ID - set if a system configuration is selected */
>>>>>  PMU_FORMAT_ATTR(configid,	"config2:32-63");
>>>>> +PMU_FORMAT_ATTR(cc_threshold,	"config3:0-11");
>>>>>
>>>>>
>>>>>  /*
>>>>> @@ -101,6 +102,7 @@ static struct attribute *etm_config_formats_attr[] = {
>>>>>  	&format_attr_preset.attr,
>>>>>  	&format_attr_configid.attr,
>>>>>  	&format_attr_branch_broadcast.attr,
>>>>> +	&format_attr_cc_threshold.attr,
>>>>>  	NULL,
>>>>>  };
>>>>>
>>>>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>>>> b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>>>> index 9d186af81ea0..9a2766f68416 100644
>>>>> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>>>> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>>>>> @@ -644,7 +644,7 @@ static int etm4_parse_event_config(struct
>>>>> coresight_device *csdev,
>>>>>  	struct etmv4_config *config = &drvdata->config;
>>>>>  	struct perf_event_attr *attr = &event->attr;
>>>>>  	unsigned long cfg_hash;
>>>>> -	int preset;
>>>>> +	int preset, cc_threshold;
>>>>>
>>>>>  	/* Clear configuration from previous run */
>>>>>  	memset(config, 0, sizeof(struct etmv4_config)); @@ -667,7 +667,15
>>>>> @@ static int etm4_parse_event_config(struct coresight_device *csdev,
>>>>>  	if (attr->config & BIT(ETM_OPT_CYCACC)) {
>>>>>  		config->cfg |= TRCCONFIGR_CCI;
>>>>>  		/* TRM: Must program this for cycacc to work */
>>>>> -		config->ccctlr = ETM_CYC_THRESHOLD_DEFAULT;
>>>>> +		cc_treshold = attr->config3 & ETM_CYC_THRESHOLD_MASK;
>>>>
>>>> Spelling again
>>>
>>> Yikes, this does not even build. Seems like I had missed the
>>> applicable config i.e CONFIG_CORESIGHT_SOURCE_ETM4X this time around.
>> Apologies.
>>>
>>>>
>>>>> +		if (cc_treshold) {
>>>>> +			if (cc_treshold < drvdata->ccitmin)
>>>>> +				config->ccctlr = drvdata->ccitmin;
>>>>> +			else
>>>>> +				config->ccctlr = cc_threshold;
>>>>> +		} else {
>>>>> +			config->ccctlr = ETM_CYC_THRESHOLD_DEFAULT;
>>>>> +		}
>>>>
>>>> Consider dropping the check against CCITMIN. There are CPUs where
>>>> CCITMIN is incorrect, e.g. see published errata 1490853 where the
>>>> value 0x100 should be 0b100 i.e. 4. On these ETMs it is possible to
>>>> set the timing threshold to four cycles instead of 256 cycles,
>>>> providing much better timing resolution. The kernel currently does
>>>> not work around this errata and uses the incorrect value of ccitmin.
>>>> If you drop the check, and trust the value provided by userspace, you
>>>> allow userspace to work around it.
>>> Why ? We could just work around the errata #1490853 while initializing
>>> the drvdata->ccitmin if that is where the problem exists.
>>
>>
>>> I dont think
>>> user space should be required to know about the erratas, and provide a
>>
>> I think that becomes less true for the tracing and PMU stuff. If you are using it you
>> likely need to know a lot about the platform you are working on anyway.
>>
>> For example right now I'm trying to upstream some metric formulas which have a
>> workaround where userspace needs to know the variant of the processor. It's not
>> possible for the kernel to do anything about it.
>>
>> In this case as it's only one known errata we could add the workaround.
>>
>> Unless we expect there to be the same issue again in the future? Or we know
>> there are already more CPUs than #1490853 mentions?
> 
> It's a common-mode failure affecting several CPUs, each with their
> own set of affected/fixed versions, so there would be several MIDRs
> to check. The ones that have been published in errata notices include:
> 
> - #1490853: Neoverse N1, fixed r4p1
> - #1490853: Cortex-A76, fixed r4p1 (n.b. same number as above)
> - #1619801: Neoverse V1, fixed r1p0
> - #1502854: Cortex-X1, fixed r1p0
> - #1491015: Cortex-A77, fixed r1p1
> 
> Hopefully that's the lot.
> 
> For this issue you don't really need to check the fix version, as the
> number you'd put in ccitmin is the same anyway.
> 
> Al
> 

It could make sense to not clamp it then. Should we at least expose
CCITMIN in sysfs so userspace can attempt to put the right value if it's
not trying to work around something? If it's not already exposed.

> 
>>
>>> right value instead.
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