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Re: [Qemu-devel] [PULL 11/24] tcg: enable thread-per-vCPU

From: Laurent Vivier
Subject: Re: [Qemu-devel] [PULL 11/24] tcg: enable thread-per-vCPU
Date: Mon, 13 Mar 2017 15:03:17 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

Le 27/02/2017 à 15:38, Alex Bennée a écrit :
> Laurent Vivier <address@hidden> writes:
>> Le 24/02/2017 à 12:20, Alex Bennée a écrit :
>>> There are a couple of changes that occur at the same time here:
>>>   - introduce a single vCPU qemu_tcg_cpu_thread_fn
>>>   One of these is spawned per vCPU with its own Thread and Condition
>>>   variables. qemu_tcg_rr_cpu_thread_fn is the new name for the old
>>>   single threaded function.
>>>   - the TLS current_cpu variable is now live for the lifetime of MTTCG
>>>     vCPU threads. This is for future work where async jobs need to know
>>>     the vCPU context they are operating in.
>>> The user to switch on multi-thread behaviour and spawn a thread
>>> per-vCPU. For a simple test kvm-unit-test like:
>>>   ./arm/run ./arm/locking-test.flat -smp 4 -accel tcg,thread=multi
>>> Will now use 4 vCPU threads and have an expected FAIL (instead of the
>>> unexpected PASS) as the default mode of the test has no protection when
>>> incrementing a shared variable.
>>> We enable the parallel_cpus flag to ensure we generate correct barrier
>>> and atomic code if supported by the front and backends. This doesn't
>>> automatically enable MTTCG until default_mttcg_enabled() is updated to
>>> check the configuration is supported.
>> This commit breaks linux-user mode:
>> debian-8 with qemu-ppc on x86_64 with ltp-full-20170116
>> cd /opt/ltp
>> ./runltp -p -l "qemu-$(date +%FT%T).log" -f /opt/ltp/runtest/syscalls -s
>> setgroups03
>> setgroups03    1  TPASS  :  setgroups(65537) fails, Size is >
>> sysconf(_SC_NGROUPS_MAX), errno=22
>> qemu-ppc: /home/laurent/Projects/qemu/include/qemu/rcu.h:89:
>> rcu_read_unlock: Assertion `p_rcu_reader->depth != 0' failed.
>> qemu-ppc: /home/laurent/Projects/qemu/include/qemu/rcu.h:89:
>> rcu_read_unlock: Assertion `p_rcu_reader->depth != 0' failed.
>> qemu-ppc: /home/laurent/Projects/qemu/include/qemu/rcu.h:89:
>> rcu_read_unlock: Assertion `p_rcu_reader->depth != 0' failed.
>> ...
> Interesting. I can only think the current_cpu change has broken it
> because most of the changes in this commit affect softmmu targets only
> (linux-user has its own run loop).
> Thanks for the report - I'll look into it.


     95b0eca Merge remote-tracking branch
'remotes/stsquad/tags/pull-mttcg-fixups-090317-1' into staging

[Tested with my HEAD on:
b1616fe Merge remote-tracking branch
'remotes/famz/tags/docker-pull-request' into staging]

I have now:

tag=setgroups03 stime=1489413401
ERROR:/home/laurent/Projects/qemu/cpu-exec.c:656:cpu_exec: assertion
failed: (cpu == current_cpu)


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