Re: Sudden slowdown of ARM emulation in master

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 26 Feb 2020 14:13:11 +0000
Alex Bennée <alex.bennee@linaro.org> wrote:

> Peter Maydell <peter.maydell@linaro.org> writes:
> 
> > On Wed, 26 Feb 2020 at 09:19, Igor Mammedov <imammedo@redhat.com> wrote:  
> >>
> >> On Wed, 26 Feb 2020 00:07:55 +0100
> >> Niek Linnenbank <nieklinnenbank@gmail.com> wrote:
> >>  
> >> > Hello Igor and Paolo,  
> >>
> >> does following hack solves issue?
> >>
> >> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> >> index a08ab11f65..ab2448c5aa 100644
> >> --- a/accel/tcg/translate-all.c
> >> +++ b/accel/tcg/translate-all.c
> >> @@ -944,7 +944,7 @@ static inline size_t size_code_gen_buffer(size_t tb_size)
> >>          /* ??? If we relax the requirement that CONFIG_USER_ONLY use the
> >>             static buffer, we could size this on RESERVED_VA, on the text
> >>             segment size of the executable, or continue to use the default.  */
> >> -        tb_size = (unsigned long)(ram_size / 4);
> >> +        tb_size = MAX_CODE_GEN_BUFFER_SIZE;
> >>  #endif
> >>      }
> >>      if (tb_size < MIN_CODE_GEN_BUFFER_SIZE) {  
> >
> > Cc'ing Richard to ask: does it still make sense for TCG
> > to pick a codegen buffer size based on the guest RAM size?  
> 
> Arguably you would never get more than ram_size * tcg gen overhead of
> active TBs at any one point although you can come up with pathological
> patterns where only a subset of pages are flushed in and out at a time.
> 
> However the backing for the code is mmap'ed anyway so surely the kernel
> can work out the kinks here. We will never allocate more than the code
> generator can generate jumps for anyway.
> 
> Looking at the SoftMMU version of alloc_code_gen_buffer it looks like
> everything now falls under the:
> 
>   # if defined(__PIE__) || defined(__PIC__)
> 
> leg so there is a bunch of code to be deleted there. The remaining
> question is what to do for linux-user because there is a bit more logic
> to deal with some corner cases on the static code generation buffer.
> 
> I'd be tempted to rename DEFAULT_CODE_GEN_BUFFER_SIZE to
> SMALL_CODE_GEN_BUFFER_SIZE and only bother with a static allocation for
> 32 bit linux-user hosts. Otherwise why not default to
> MAX_CODE_GEN_BUFFER_SIZE on 64 bit systems and let the kernel deal with
> it?

*-user call
  tcg_exec_init(0);
which in in the end results in
  DEFAULT_CODE_GEN_BUFFER_SIZE -> DEFAULT_CODE_GEN_BUFFER_SIZE_1

so for *-user cases we can just always call
   code_gen_alloc(DEFAULT_CODE_GEN_BUFFER_SIZE)

> > (We should fix the regression anyway, but it surprised me
> > slightly to find a config detail of the guest machine being
> > used here.)
> >
> > thanks
> > -- PMM  
> 
>