Re: [PATCH 0/8] Re-write PPC64 PMU instruction count using TCG Ops

[Richard Henderson]

On 12/22/21 5:45 AM, Daniel Henrique Barboza wrote:
> Hi,
>
> Two days ago Richard Henderson reported test failures with Avocado and
> powernv8/9 due to timeouts [1]. The culprit ended up to be commit , a
> commit where I introduced PMU instruction counting for TCG PPC64.
>
> For a reason that is still unclear to me these Avocado powernv tests are
> suffering a huge performance impact after that patch, something that I
> didn't verify in any other scenario I've tested. So one alternative to
> fix the situation is to understand this difference and try to solve it,
> which can take some time.
>   
> Another alternative is to optimize the code introduced by that commit.
> Today the instruction count is done by a TCG helper that is called after
> each TB exit. I was aware that calling a helper frequently isn't
> optimal, but that got the job done and didn't  hindered the use of
> pSeries and powernv machines.  Well, until [1] at least.
>
> This series rewrites the PMU instruction counting using TCG Ops instead
> of a TCG helper. To do that we needed to write in TCG Ops not only the
> logic for increment the counters but also the logic to detect counter
> overflows.
>
> A lot of code was added but the performance improvement is noticeable.
> Using my local machine I did some test runs with the 2 Avocado powernv
> tests that are timing out at this moment:

You generate a *lot* of inline code here.  Way too much, actually.

If you can get this performance improvement with this reorg, it merely means that your 
original C algorithm was poor.  The compiler should have been able to do better.

I've tested this theory here and...

> - failing Avocado powernv tests with current master:
>
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: 
> PASS (70.17 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: 
> PASS (70.90 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: 
> PASS (70.81 s)
>   
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: PASS (75.62 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: 
> PASS (69.79 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: 
> PASS (72.33 s)

boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: PASS (75.73 s)
boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: PASS (80.20 s)

> - after this series:
>
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: 
> PASS (39.90 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: 
> PASS (38.25 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: 
> PASS (37.99 s)
>
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: 
> PASS (43.17 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: 
> PASS (43.64 s)
>   (1/1) tests/avocado/boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: 
> PASS (44.21 s)

boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: PASS (39.66 s)
boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: PASS (43.02 s)

BTW, pre-power8-pmu, 29c4a3363b:

boot_linux_console.py:BootLinuxConsole.test_ppc_powernv8: PASS (36.62 s)
boot_linux_console.py:BootLinuxConsole.test_ppc_powernv9: PASS (39.69 s)

I'll post my series shortly.


r~