Re: [PATCH] mips64el-softmmu: Enable MTTCG

["Jiaxun Yang" <jiaxun.yang@flygoat.com>]

在2024年6月3日六月 下午7:15，Pierrick Bouvier写道：
> On 5/11/24 13:26, Jiaxun Yang wrote:
>> MTTCG was disabled in a092a9554771 ("configure: disable MTTCG
>> for MIPS guests") due to test case instability.
>> 
>> I was able to reproduce this issue with in latest QEMU and look
>> into reason behind that.
>> 
>> What actually happend is kernel's CP0 timer synchronisation
>> mechanism assumed a consistent latency in memory access between
>> cores, which TCG can't guarantee. Thus there is a huge drift in
>> count value between cores, and in early kernel versions CP0 timer
>> is always used as sched_clock.
>> 
>> sched_clock drift back on some cores triggered RCU watchdog in
>> some extreme cases.
>> 
>> This can be resolved by setting clocksource to MIPS, which allows
>> clocksource to drift together with sched_clock. However this will
>> leed to other problems after boot.
>> 
>> Another option would beupdating kernel to later version, which
>> will use GIC as sched_clock.
>> 
>> In non-MTTCG build, the execution is slow enough so kernel won't
>> observe back drifts.
>> 
>> Test results:
>> 
>> With clocksource=MIPS
>> ```
>>   ~/tmp/retry/retry.py -n 100 -c -- ./qemu-system-mips64el \
>>      -display none -vga none -serial mon:stdio \
>>      -machine malta -kernel ./vmlinux-4.7.0-rc1.I6400 \
>>      -cpu I6400 -smp 8 -vga std \
>>      -append "printk.time=0 clocksource=MIPS console=tty0 console=ttyS0 panic=-1" \
>>      --no-reboot
>> 
>> 100, 0, PASS, 5.258126, 100, 100, -
>> Results summary:
>> 0: 100 times (100.00%), avg time 6.508 (55.53 varience/7.45 deviation)
>> Ran command 100 times, 100 passes
>> ```
>> 
>> With linux-next:
>> ```
>>   ~/tmp/retry/retry.py -n 100 -c -- ./qemu-system-mips64el \
>>      -display none -vga none -serial mon:stdio \
>>      -machine malta -kernel ~/linux-next/vmlinux \
>>      -cpu I6400 -smp 8 -vga std \
>>      -append "printk.time=0 console=tty0 console=ttyS0 panic=-1" \
>>      --no-reboot
>> 
>> 100, 0, PASS, 4.507921, 100, 100, -
>> Results summary:
>> 0: 100 times (100.00%), avg time 4.233 (0.04 varience/0.21 deviation)
>> Ran command 100 times, 100 passes
>> ```
>> 
>> Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
>> ---
>> I'll leave the test case alone as it's already marked as QEMU_TEST_FLAKY_TESTS
>> ---
>>   configs/targets/mips64el-softmmu.mak | 1 +
>>   1 file changed, 1 insertion(+)
>> 
>> diff --git a/configs/targets/mips64el-softmmu.mak b/configs/targets/mips64el-softmmu.mak
>> index 8d9ab3ddc4b1..199b1d909a7d 100644
>> --- a/configs/targets/mips64el-softmmu.mak
>> +++ b/configs/targets/mips64el-softmmu.mak
>> @@ -1,3 +1,4 @@
>>   TARGET_ARCH=mips64
>>   TARGET_BASE_ARCH=mips
>> +TARGET_SUPPORTS_MTTCG=y
>>   TARGET_NEED_FDT=y
>> 
>> ---
>> base-commit: 248f6f62df073a3b4158fd0093863ab885feabb5
>> change-id: 20240511-mips_mttcg-47a6b19074b3
>> 
>> Best regards,
>
> Hi Jiaxun,
> Thanks for your analysis!
>
> We should see to update concerned test in another series.
> I'm not sure which way is preferred between updating kernel used or 
> changing current command line.
>
> Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>

I think updating kernel can prevent further problems.
Will test with latest Debian kernel.

Thanks!
-- 
- Jiaxun