linux-kernel - [BUG REPORT] perf tools: x86_64: weight column displays odd memory latency data

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Date:   Sun, 22 May 2022 22:14:49 +0000
From:   Tyson Thomas <tyson.thomas@...ney.edu.au>
To:     "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>
Subject: [BUG REPORT] perf tools: x86_64: weight column displays odd memory
 latency data

Hi Kan, Linux-Perf Team

I have observed some odd behaviour within perf when using perf-mem. Specifically the reported latency under the weight column appears to be unreasonably high.

Here is a given sample from a recent test, I find that some of the  latencies are close to an unsigned short and I cannot seem to be understand why that would be outside of it being an issue with the perf events.

This can be replicated using a NAS benchmark, specifically cg.D.

I observe the following results in perf mem report (just getting the top 10 results)

 0.02% ,62515 ,L1 or L1 hit 
 0.02% ,54048 ,L1 or L1 hit 
 0.02% ,52206 ,L1 or L1 hit 
 0.02% ,49831 ,L1 or L1 hit 
 0.02% ,49056 ,Local RAM or RAM hit 
 0.01% ,40666 ,LFB or LFB hit 
 0.01% ,38080 ,L1 or L1 hit 
 0.01% ,36772 ,L1 or L1 hit 
 0.01% ,36729 ,LFB or LFB hit 
 0.01% ,27101 ,LFB or LFB hit

Is it possible for someone to shed some light on this or am I misunderstanding how the weight column is used here?
This appears to have been an issue on 5.4, 5.10 and 5.15. I am looking into seeing if it is still present in 5.17 and 5.18.

I've also tried this on different Intel CPUs such as Intel Xeon 6230, i5-1135G7, Intel Xeon 6330

Any insight or help would be appreciated,
Tyson