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Date:   Tue, 26 Apr 2022 11:18:08 +0200
From:   Robert Richter <rrichter@amd.com>
To:     Ravi Bangoria <ravi.bangoria@amd.com>
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Subject: Re: [PATCH 1/6] perf/amd/ibs: Add support for L3 miss filtering
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On 25.04.22 10:13:18, Ravi Bangoria wrote:
> IBS L3 miss filtering works by tagging an instruction on IBS counter
> overflow and generating an NMI if the tagged instruction causes an L3
> miss. Samples without an L3 miss are discarded and counter is reset
> with random value (between 1-15 for fetch pmu and 1-127 for op pmu).
> This helps in reducing sampling overhead when user is interested only
> in such samples. One of the use case of such filtered samples is to
> feed data to page-migration daemon in tiered memory systems.
> 
> Add support for L3 miss filtering in IBS driver via new pmu attribute
> "l3missonly". Example usage:
> 
>   # perf record -a -e ibs_op/l3missonly=1/ --raw-samples sleep 5
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>
> ---
>  arch/x86/events/amd/ibs.c         | 42 ++++++++++++++++++++++---------
>  arch/x86/include/asm/perf_event.h |  3 +++
>  2 files changed, 33 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 9739019d4b67..a5303d62060c 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -520,16 +520,12 @@ static void perf_ibs_read(struct perf_event *event) { }
>  
>  PMU_FORMAT_ATTR(rand_en,	"config:57");
>  PMU_FORMAT_ATTR(cnt_ctl,	"config:19");
> +PMU_EVENT_ATTR_STRING(l3missonly, fetch_l3missonly, "config:59");
> +PMU_EVENT_ATTR_STRING(l3missonly, op_l3missonly, "config:16");
>  
> -static struct attribute *ibs_fetch_format_attrs[] = {
> -	&format_attr_rand_en.attr,
> -	NULL,
> -};
> -
> -static struct attribute *ibs_op_format_attrs[] = {
> -	NULL,	/* &format_attr_cnt_ctl.attr if IBS_CAPS_OPCNT */
> -	NULL,
> -};
> +/* size = nr attrs plus NULL at the end */
> +static struct attribute *ibs_fetch_format_attrs[3];
> +static struct attribute *ibs_op_format_attrs[3];

Define a macro for the array size.

>  
>  static struct perf_ibs perf_ibs_fetch = {
>  	.pmu = {
> @@ -759,9 +755,9 @@ static __init int perf_ibs_pmu_init(struct perf_ibs *perf_ibs, char *name)
>  	return ret;
>  }
>  
> -static __init void perf_event_ibs_init(void)
> +static __init void perf_ibs_fetch_prepare(void)

Since this actually initializes the pmu, let's call that
perf_ibs_fetch_init().

For low level init functions it would be good to keep track of the
return code even if it is later not evaluated. So these kind of
function should return an error code.

>  {
> -	struct attribute **attr = ibs_op_format_attrs;
> +	struct attribute **format_attrs = perf_ibs_fetch.format_attrs;

I think we could keep this short here with 'attr'.

>  
>  	/*
>  	 * Some chips fail to reset the fetch count when it is written; instead
> @@ -773,11 +769,22 @@ static __init void perf_event_ibs_init(void)
>  	if (boot_cpu_data.x86 == 0x19 && boot_cpu_data.x86_model < 0x10)
>  		perf_ibs_fetch.fetch_ignore_if_zero_rip = 1;
>  
> +	*format_attrs++ = &format_attr_rand_en.attr;
> +	if (ibs_caps & IBS_CAPS_ZEN4IBSEXTENSIONS) {
> +		perf_ibs_fetch.config_mask |= IBS_FETCH_L3MISSONLY;
> +		*format_attrs++ = &fetch_l3missonly.attr.attr;
> +	}

You should also write the terminating NULL pointer here, though the
mem is preinitialized zero.

> +
>  	perf_ibs_pmu_init(&perf_ibs_fetch, "ibs_fetch");
> +}
> +
> +static __init void perf_ibs_op_prepare(void)
> +{
> +	struct attribute **format_attrs = perf_ibs_op.format_attrs;
>  
>  	if (ibs_caps & IBS_CAPS_OPCNT) {
>  		perf_ibs_op.config_mask |= IBS_OP_CNT_CTL;
> -		*attr++ = &format_attr_cnt_ctl.attr;
> +		*format_attrs++ = &format_attr_cnt_ctl.attr;
>  	}
>  
>  	if (ibs_caps & IBS_CAPS_OPCNTEXT) {
> @@ -786,7 +793,18 @@ static __init void perf_event_ibs_init(void)
>  		perf_ibs_op.cnt_mask    |= IBS_OP_MAX_CNT_EXT_MASK;
>  	}
>  
> +	if (ibs_caps & IBS_CAPS_ZEN4IBSEXTENSIONS) {
> +		perf_ibs_op.config_mask |= IBS_OP_L3MISSONLY;
> +		*format_attrs++ = &op_l3missonly.attr.attr;
> +	}
> +
>  	perf_ibs_pmu_init(&perf_ibs_op, "ibs_op");
> +}

Same for this function: *_init(), error code, attrs, terminating NULL.

> +
> +static __init void perf_event_ibs_init(void)
> +{
> +	perf_ibs_fetch_prepare();
> +	perf_ibs_op_prepare();
>  
>  	register_nmi_handler(NMI_LOCAL, perf_ibs_nmi_handler, 0, "perf_ibs");
>  	pr_info("perf: AMD IBS detected (0x%08x)\n", ibs_caps);

The function is now small enough to be squashed into amd_ibs_init().

-Robert