linux-kernel - Re: [PATCH v2 2/5] perf/x86/intel: Record branch type

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Message-ID: <20170407152031.k2auigfkj7ek4suo@hirez.programming.kicks-ass.net>
Date:   Fri, 7 Apr 2017 17:20:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jin Yao <yao.jin@...ux.intel.com>
Cc:     acme@...nel.org, jolsa@...nel.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 2/5] perf/x86/intel: Record branch type

On Fri, Apr 07, 2017 at 06:47:43PM +0800, Jin Yao wrote:
> Perf already has support for disassembling the branch instruction
> and using the branch type for filtering. The patch just records
> the branch type in perf_branch_entry.
> 
> Before recording, the patch converts the x86 branch classification
> to common branch classification and compute for checking if the
> branches cross 4K or 2MB areas. It's an approximate computing for
> crossing 4K page or 2MB page.

The changelog is completely empty of rationale. Why do we care?

Not having the binary is a very bad reason; you can't do much of
anything if that's missing.


> @@ -923,6 +933,84 @@ static int branch_type(unsigned long from, unsigned long to, int abort)
>  	return ret;
>  }
>  
> +static int
> +common_branch_type(int type, u64 from, u64 to)
> +{
> +	int ret;
> +
> +	type = type & (~(X86_BR_KERNEL | X86_BR_USER));
> +
> +	switch (type) {
> +	case X86_BR_CALL:
> +	case X86_BR_ZERO_CALL:
> +		ret = PERF_BR_CALL;
> +		break;
> +
> +	case X86_BR_RET:
> +		ret = PERF_BR_RET;
> +		break;
> +
> +	case X86_BR_SYSCALL:
> +		ret = PERF_BR_SYSCALL;
> +		break;
> +
> +	case X86_BR_SYSRET:
> +		ret = PERF_BR_SYSRET;
> +		break;
> +
> +	case X86_BR_INT:
> +		ret = PERF_BR_INT;
> +		break;
> +
> +	case X86_BR_IRET:
> +		ret = PERF_BR_IRET;
> +		break;
> +
> +	case X86_BR_IRQ:
> +		ret = PERF_BR_IRQ;
> +		break;
> +
> +	case X86_BR_ABORT:
> +		ret = PERF_BR_FAR_BRANCH;
> +		break;
> +
> +	case X86_BR_JCC:
> +		if (to > from)
> +			ret = PERF_BR_JCC_FWD;
> +		else
> +			ret = PERF_BR_JCC_BWD;
> +		break;

This seems like superfluous information; we already get to and from, so
this comparison is pointless.

The rest looks like something you can simpler implement using a lookup
table.

> +
> +	case X86_BR_JMP:
> +		ret = PERF_BR_JMP;
> +		break;
> +
> +	case X86_BR_IND_CALL:
> +		ret = PERF_BR_IND_CALL;
> +		break;
> +
> +	case X86_BR_IND_JMP:
> +		ret = PERF_BR_IND_JMP;
> +		break;
> +
> +	default:
> +		ret = PERF_BR_NONE;
> +	}
> +
> +	return ret;
> +}
> +
> +static bool
> +cross_area(u64 addr1, u64 addr2, int size)
> +{
> +	u64 align1, align2;
> +
> +	align1 = addr1 & ~(size - 1);
> +	align2 = addr2 & ~(size - 1);
> +
> +	return (align1 != align2) ? true : false;
> +}
> +
>  /*
>   * implement actual branch filter based on user demand.
>   * Hardware may not exactly satisfy that request, thus
> @@ -939,7 +1027,8 @@ intel_pmu_lbr_filter(struct cpu_hw_events *cpuc)
>  	bool compress = false;
>  
>  	/* if sampling all branches, then nothing to filter */
> -	if ((br_sel & X86_BR_ALL) == X86_BR_ALL)
> +	if (((br_sel & X86_BR_ALL) == X86_BR_ALL) &&
> +	    ((br_sel & X86_BR_TYPE_SAVE) != X86_BR_TYPE_SAVE))
>  		return;
>  
>  	for (i = 0; i < cpuc->lbr_stack.nr; i++) {
> @@ -960,6 +1049,21 @@ intel_pmu_lbr_filter(struct cpu_hw_events *cpuc)
>  			cpuc->lbr_entries[i].from = 0;
>  			compress = true;
>  		}
> +
> +		if ((br_sel & X86_BR_TYPE_SAVE) == X86_BR_TYPE_SAVE) {
> +			cpuc->lbr_entries[i].type = common_branch_type(type,
> +								       from,
> +								       to);
> +			if (cross_area(from, to, AREA_2M))
> +				cpuc->lbr_entries[i].cross = PERF_BR_CROSS_2M;
> +			else if (cross_area(from, to, AREA_4K))
> +				cpuc->lbr_entries[i].cross = PERF_BR_CROSS_4K;
> +			else
> +				cpuc->lbr_entries[i].cross = PERF_BR_CROSS_NONE;

This again is superfluous information; it is already fully contained in
to and from, which we have.

> +		} else {
> +			cpuc->lbr_entries[i].type = PERF_BR_NONE;
> +			cpuc->lbr_entries[i].cross = PERF_BR_CROSS_NONE;
> +		}
>  	}
>  
>  	if (!compress)
> -- 
> 2.7.4
>