Re: [PATCH V4 4/7] perf/x86/intel: Support LBR event logging

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2023-10-19 6:52 a.m., Peter Zijlstra wrote:
> On Wed, Oct 04, 2023 at 11:40:41AM -0700, kan.liang@linux.intel.com wrote:
> 
>> +#define ARCH_LBR_EVENT_LOG_WIDTH	2
>> +#define ARCH_LBR_EVENT_LOG_MASK		0x3
> 
> event log ?

That's the name in the Intel spec. I will change it to the name used in
Linux and add a comment to map the name event log to the name branch
counter.

> 
> 
>> +static __always_inline void intel_pmu_update_lbr_event(u64 *lbr_events, int idx, int pos)
>> +{
>> +	u64 logs = *lbr_events >> (LBR_INFO_EVENTS_OFFSET +
>> +				   idx * ARCH_LBR_EVENT_LOG_WIDTH);
>> +
>> +	logs &= ARCH_LBR_EVENT_LOG_MASK;
>> +	*lbr_events |= logs << (pos * ARCH_LBR_EVENT_LOG_WIDTH);
>> +}
>> +
>> +/*
>> + * The enabled order may be different from the counter order.
>> + * Update the lbr_events with the enabled order.
>> + */
>> +static void intel_pmu_lbr_event_reorder(struct cpu_hw_events *cpuc,
>> +					struct perf_event *event)
>> +{
>> +	int i, j, pos = 0, enabled[X86_PMC_IDX_MAX];
>> +	struct perf_event *leader, *sibling;
>> +
>> +	leader = event->group_leader;
>> +	if (branch_sample_counters(leader))
>> +		enabled[pos++] = leader->hw.idx;
>> +
>> +	for_each_sibling_event(sibling, leader) {
>> +		if (!branch_sample_counters(sibling))
>> +			continue;
>> +		enabled[pos++] = sibling->hw.idx;
>> +	}
> 
> Ok, so far so good: enabled[x] = y, is a mapping of hardware index (y)
> to group order (x).
> 
> Although I would perhaps name that order[] instead of enabled[].

Sure

> 
>> +
>> +	if (!pos)
>> +		return;
> 
> How would we ever get here if this is the case?

It should be a bug. I will use a WARN_ON_ONCE() to replace it.

> 
>> +
>> +	for (i = 0; i < cpuc->lbr_stack.nr; i++) {
>> +		for (j = 0; j < pos; j++)
>> +			intel_pmu_update_lbr_event(&cpuc->lbr_events[i], enabled[j], j);
> 
> But this confuses me... per that function it:
> 
>  - extracts counter value for enabled[j] and,
>  - or's it into the same variable at j
> 
> But what if j is already taken by something else?
> 
> That is, suppose enabled[] = {3,2,1,0}, and lbr_events = 11 10 01 00
> 
> Then: for (j) intel_pmu_update_lbt_event(&lbr_event, enabled[j], j);
> 
> 0: 3->0, 11 10 01 00 -> 11 10 01 11
> 1: 2->1, 11 10 01 11 -> 11 10 11 11
> 2: 1->2, 11 10 11 11 -> 11 11 11 11
> 
> 
> 
>> +
>> +		/* Clear the original counter order */
>> +		cpuc->lbr_events[i] &= ~LBR_INFO_EVENTS;
>> +	}
>> +}
> 
> Would not something like:
> 
> 	src = lbr_events[i];
> 	dst = 0;
> 	for (j = 0; j < pos; j++) {
> 		cnt = (src >> enabled[j]*2) & 3;
> 		dst |= cnt << j*2
> 	}
> 	lbr_events[i] = dst;
> 
> be *FAR* clearer, and actually work?

The original LBR event data is saved at offset 32 of LBR_INFO register.
In get_lbr_events(), the data was simply copied to the offset 32 of
cpuc->lbr_events.

The intel_pmu_update_lbr_event() reorders the value and saves it
starting from the offset 0.

I agree it's hard to read since it combines the src and dst into the
same variable.

I will use the suggested code and also update the get_lbr_events().

	cpuc->lbr_events[i] = (info >> 32) & LBR_INFO_EVENTS;

Thanks,
Kan