Re: [PATCH] perf/amd/ibs: add phy_addr_only software filter for ibs_op

[Ravi Bangoria <ravi.bangoria@amd.com>]

Huang,

>> Can you please confirm if those additional samples are genuinely valid.
> 
> Yes.  
> To answer this properly I re-ran the mysql test,
> and verified the physical address of every captured sample against
> the System RAM ranges in /proc/iomem.
> 
> Setup: 384-CPU Hygon box, v7.3-based kernel, MySQL 8 under sysbench
> oltp_read_only, 400 threads.  Each round is a fresh 60s
> sysbench run; after 15s of warmup, perf records ibs_op for 3s:
> 
>   $ perf record -e ibs_op/phy_addr_only=1/ -c 100000 --phys-data \
>         -p <mysqld>                    # my patch
>   $ perf record -e ibs_op// --phys-data --filter 'phys_addr != 0' \
>         -c 100000 -p <mysqld>          # your suggestion
> 
> Rounds alternate p1 -> b1 -> p2 -> b2 -> p3 -> b3 so both methods see
> the same load conditions.  Each record's phys_addr is then dumped with
> "perf script -F pid,tid,phys_addr" and bisect-checked against the
> System RAM ranges (read from /proc/iomem as root).
> 
>                      samples    phys_addr==0    non-zero in
>     round  filter    captured   (valid=1,MSR=0)  System RAM
>     -----  --------  ---------  ---------------  -----------
>      p1    phy_only  15710042        0.10 %          98.61 %
>      b1    bpf       14041851        0.00 %          98.55 %
>      p2    phy_only  15580497        0.10 %          98.56 %
>      b2    bpf       15416187        0.00 %          98.58 %
>      p3    phy_only  15732582        0.10 %          98.59 %
>      b3    bpf       15447397        0.00 %          98.58 %
> 
> Two observations:
> 1. The validity ratio is identical for both methods.  Of the non-zero
>    addresses phy_addr_only delivers, 98.6 % fall inside System RAM --
>    the same ratio the BPF-filtered set shows. 
> 
> 2. The only set-level difference is 0.10 % of samples where
>    dc_phy_addr_valid is set but IBSDCPHYSAD reads 0. 

The BPF filter is giving more accurate data here because neither kernel
nor userspace should access the first physical page:

  $ sudo head -1 /proc/iomem
  00000000-00000fff : Reserved

Thanks,
Ravi