Re: tcg_flush_jmp_cache replacing qatomic_set loop with memset

["Richard W.M. Jones" <rjones@redhat.com>]

On Mon, Oct 16, 2023 at 04:43:36PM +0100, Richard W.M. Jones wrote:
> Hey Paolo,
> 
> Quick question.  I'm sure the transformation below is *not* correct,
> because it doesn't preserve the invariant of the lockless structure.
> Is there a way to do this while maintaining correctness?  For example
> putting barrier() after memset?  (Note I'm also zeroing .pc which may
> be a problem.)

Alright so ignore this question :-(

After inspecting the assembly on x86-64, I can see the qatomic_set
simply expands to a regular store (actually looks like it is
unrolled by 2):

  716340:       48 c7 00 00 00 00 00    movq   $0x0,(%rax)
  716347:       48 c7 40 10 00 00 00    movq   $0x0,0x10(%rax)
  71634e:       00 
  71634f:       48 83 c0 20             add    $0x20,%rax
  716353:       48 39 d0                cmp    %rdx,%rax
  716356:       75 e8                   jne    716340 <tcg_flush_jmp_cache+0x20>

My memset version was twice as fast because it used some avx
instructions.

I guess this would do something more fancy on aarch64 host ...

Rich.

> The background to this is that I've been playing around with the very
> hot tb_lookup function.  Increasing the size of the jump cache (which
> hasn't changed since, erm, 2005!), looks like it could improve
> performance, plus a few other changes which I'm playing with.  However
> increasing the size causes profiles to be dominated by the loop in
> tcg_flush_jmp_cache, presumably because of all those serialized atomic ops.
> 
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index 8cb6ad3511..6a21b3dba8 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -796,9 +796,7 @@ void tcg_flush_jmp_cache(CPUState *cpu)
>          return;
>      }
>  
> -    for (int i = 0; i < TB_JMP_CACHE_SIZE; i++) {
> -        qatomic_set(&jc->array[i].tb, NULL);
> -    }
> +    memset(jc->array, 0, TB_JMP_CACHE_SIZE * sizeof jc->array[0]);
>  }
>  
>  /* This is a wrapper for common code that can not use CONFIG_SOFTMMU */
> 
> Rich.
> 
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> nbdkit - Flexible, fast NBD server with plugins
> https://gitlab.com/nbdkit/nbdkit

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit