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

From: Alex Bennée
Subject: Re: [Qemu-devel] [PULL 11/24] tcg: enable thread-per-vCPU
Date: Mon, 27 Feb 2017 14:38:21 +0000
User-agent: mu4e 0.9.19; emacs 25.2.7

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.

Alex Bennée

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