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From: "Pavel Dovgalyuk" <dovgaluk@ispras.ru>
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Date: Tue, 22 Jan 2019 14:31:25 +0300
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Subject: Re: [Qemu-devel] Bottom halves VS timers
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To: 'Paolo Bonzini' <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: peter.maydell@linaro.org, stefanha@redhat.com, famz@redhat.com, kwolf@redhat.com

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 22/01/19 08:15, Pavel Dovgalyuk wrote:
> > Scheduling new BH does not create a signal for the vCPU to suspend the
> > execution and process the main loop events (in record/replay we
> > synchronize these two threads).
> 
> I think the bug is that in rr mode it should do so and cause a cpu_exit.

Right. I combined adding cpu_exit (into our private version) and increasing
an icount command-line parameter.

>  If replace these bottom halves with timers, other bottom halves from
> the block layer may have the same issue; if you replace _all_ bottom
> halves with timers you have performance issues because bottom halves try
> very hard to avoid locks.

I see.

> Unfortunately this is getting really ugly...  Basically, rr is
> reintroducing all the complications with iothread_requesting_mutex that
> were removed with mtTCG.

True. There are too many non-synchronized entities: bottom halves,
worker threads, iothreads, timers...
But RR is only possible when we have kind of synchronization objects for them.

Pavel Dovgalyuk