linux-kernel - Re: [PATCH V3 3/3] perf tools: Construct LBR call chain

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Message-ID: <87d28lqb35.fsf@sejong.aot.lge.com>
Date:	Tue, 18 Nov 2014 15:25:34 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	kan.liang@...el.com
Cc:	acme@...nel.org, jolsa@...hat.com, a.p.zijlstra@...llo.nl,
	eranian@...gle.com, linux-kernel@...r.kernel.org, mingo@...hat.com,
	paulus@...ba.org, ak@...ux.intel.com
Subject: Re: [PATCH V3 3/3] perf tools: Construct LBR call chain

On Fri, 14 Nov 2014 08:44:12 -0500, kan liang wrote:
> +		/* LBR only affects the user callchain */
> +		if (i != chain_nr) {
> +			struct branch_stack *lbr_stack = sample->branch_stack;
> +			int lbr_nr = lbr_stack->nr;
> +			/*
> +			 * LBR callstack can only get user call chain.
> +			 * The mix_chain_nr is kernel call chain
> +			 * number plus LBR user call chain number.
> +			 * i is kernel call chain number,
> +			 * 1 is PERF_CONTEXT_USER,
> +			 * lbr_nr + 1 is the user call chain number.
> +			 * For details, please refer to the comments
> +			 * in callchain__printf
> +			 */
> +			int mix_chain_nr = i + 1 + lbr_nr + 1;
> +
> +			if (mix_chain_nr > PERF_MAX_STACK_DEPTH) {
> +				pr_warning("corrupted callchain. skipping...\n");
> +				return 0;
> +			}

I'm not sure whether this is really a corrupted callchain.  If a single
chain is greater than the max it should be corrupted, but we're now
summing up with other values..

What about checking callchain_nr and lbr_nr separately or mix_nr with
2*max?

Thanks,
Namhyung
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