Re: [PATCH 2/7] aio-posix: move RCU_READ_LOCK() into run_poll_handlers()

[Stefan Hajnoczi]

On Thu, Mar 05, 2020 at 06:15:36PM +0100, Paolo Bonzini wrote:
> On 05/03/20 18:08, Stefan Hajnoczi wrote:
> >  
> > +    /*
> > +     * Optimization: ->io_poll() handlers often contain RCU read critical
> > +     * sections and we therefore see many rcu_read_lock() -> 
> > rcu_read_unlock()
> > +     * -> rcu_read_lock() -> ... sequences with expensive memory
> > +     * synchronization primitives.  Make the entire polling loop an RCU
> > +     * critical section because nested rcu_read_lock()/rcu_read_unlock() 
> > calls
> > +     * are cheap.
> > +     */
> > +    RCU_READ_LOCK_GUARD();
> > +
> 
> Looks good, but I suggest that you compile with --enable-membarrier as
> that makes RCU critical sections basically free.

Interesting, --enable-membarrier decreases performance from 105k to 97k
IOPS in the NVMe latency benchmark that I'm running.

Stefan

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