Re: [Qemu-devel] [RFC 33/38] cpu: introduce cpu_tcg_sched_work to run work while other CPUs sleep

[Paolo Bonzini]

On 23/08/2015 17:24, Emilio G. Cota wrote:
> This is similar in intent to the async_safe_work mechanism. The main
> differences are:
> 
> - Work is run on a single CPU thread *after* all others are put to sleep
> 
> - Sleeping threads are woken up by the worker thread upon completing its job
> 
> - A flag as been added to tcg_ctx so that only one thread can schedule
>   work at a time. The flag is checked every time tb_lock is acquired.
> 
> - Handles the possibility of CPU threads being created after the existing
>   CPUs are put to sleep. This is easily triggered with many threads on
>   a many-core host in usermode.
> 
> - Works for both softmmu and usermode
> 
> Signed-off-by: Emilio G. Cota <address@hidden>

I think this is a duplicate of the existing run_on_cpu code.  If needed
in user-mode emulation, it should be extracted out of cpus.c.

Also I think it is dangerous (prone to deadlocks) to wait for other CPUs
with synchronize_cpu and condvar.  I would much rather prefer to _halt_
the CPUs if there is pending work, and keep it halted like this:

 static inline bool cpu_has_work(CPUState *cpu)
 {
     CPUClass *cc = CPU_GET_CLASS(cpu);

+    if (tcg_ctx.tb_ctx.tcg_has_work) {
+        return false;
+    }
     g_assert(cc->has_work);
     return cc->has_work(cpu);
 }

You can then run flush_queued_work from linux-user/main.c (and
bsd-user/main.c) when cpu_exec returns EXCP_HALTED.

Paolo