Re: [PATCH 0/3] Reorg ppc64 pmu insn counting

[Alex Bennée]

Daniel Henrique Barboza <danielhb413@gmail.com> writes:

> On 12/23/21 00:01, Richard Henderson wrote:
>> In contrast to Daniel's version, the code stays in power8-pmu.c,
>> but is better organized to not take so much overhead.
>> Before:
>>      32.97%  qemu-system-ppc  qemu-system-ppc64   [.] pmc_get_event
>>      20.22%  qemu-system-ppc  qemu-system-ppc64   [.] helper_insns_inc
>>       4.52%  qemu-system-ppc  qemu-system-ppc64   [.] 
>> hreg_compute_hflags_value
>>       3.30%  qemu-system-ppc  qemu-system-ppc64   [.] helper_lookup_tb_ptr
>>       2.68%  qemu-system-ppc  qemu-system-ppc64   [.] tcg_gen_code
>>       2.28%  qemu-system-ppc  qemu-system-ppc64   [.] cpu_exec
>>       1.84%  qemu-system-ppc  qemu-system-ppc64   [.] pmu_insn_cnt_enabled
>> After:
>>       8.42%  qemu-system-ppc  qemu-system-ppc64   [.]
>> hreg_compute_hflags_value
>>       6.65%  qemu-system-ppc  qemu-system-ppc64   [.] cpu_exec
>>       6.63%  qemu-system-ppc  qemu-system-ppc64   [.] helper_insns_inc
>> 
>
> Thanks for looking this up. I had no idea the original C code was that slow.
>
<snip>
>
> With that in mind I decided to post a new version of my TCG rework, with less 
> repetition and
> a bit more concise, to have an alternative that can be used upstream to fix 
> the Avocado tests.
> Meanwhile I'll see if I can get your reorg working with all EBB tests we 
> need. All things
> equal - similar performance, all EBB tests passing - I'd rather stay with 
> your C code than my
> TCG rework since yours doesn't rely on TCG Ops knowledge to maintain
> it.

Reading this series did make me wonder if we need a more generic service
from the TCG for helping with "internal" instrumentation needed for
things like decent PMU emulation. We haven't gone as much for it in ARM
yet but it would be nice to. It would be even nicer if such a facility
could be used by stuff like icount as well so we don't end up doing the
same thing twice.

>
>
> Thanks,
>
>
> Daniel
>
>
> [1] 
> https://github.com/torvalds/linux/tree/master/tools/testing/selftests/powerpc/pmu/ebb
> [2] https://lists.gnu.org/archive/html/qemu-devel/2021-12/msg00073.html
>
>> r~
>> Richard Henderson (3):
>>    target/ppc: Cache per-pmc insn and cycle count settings
>>    target/ppc: Rewrite pmu_increment_insns
>>    target/ppc: Use env->pnc_cyc_cnt
>>   target/ppc/cpu.h         |   3 +
>>   target/ppc/power8-pmu.h  |  14 +--
>>   target/ppc/cpu_init.c    |   1 +
>>   target/ppc/helper_regs.c |   2 +-
>>   target/ppc/machine.c     |   2 +
>>   target/ppc/power8-pmu.c  | 230 ++++++++++++++++-----------------------
>>   6 files changed, 108 insertions(+), 144 deletions(-)
>> 


-- 
Alex Bennée