Re: Sudden slowdown of ARM emulation in master

["Alex Bennée" <alex.bennee@linaro.org>]

Igor Mammedov <imammedo@redhat.com> writes:

> 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)
<snip>

I've gone for a variation of that, coming to a mailing list near you
real soon now ;-)

-- 
Alex Bennée