Re: [PATCH 3/4] mm/memcg: Add a local_lock_t for IRQ and TASK object.

[Michal Hocko <mhocko@suse.com>]

On Thu 03-02-22 10:54:07, Sebastian Andrzej Siewior wrote:
> On 2022-02-01 16:29:35 [+0100], Michal Hocko wrote:
> > > > Sorry, I know that this all is not really related to your work but if
> > > > the original optimization is solely based on artificial benchmarks then
> > > > I would rather drop it and also make your RT patchset easier.
> > > 
> > > Do you have any real-world benchmark in mind? Like something that is
> > > already used for testing/ benchmarking and would fit here?
> > 
> > Anything that even remotely resembles a real allocation heavy workload.
> 
> So I figured out that build the kernel as user triggers the allocation
> path in_task() and in_interrupt(). I booted a PREEMPT_NONE kernel and
> run "perf stat -r 5 b.sh" where b.sh unpacks a kernel and runs a
> allmodconfig build on /dev/shm. The slow disk should not be a problem.
> 
> With the optimisation:
> |  Performance counter stats for './b.sh' (5 runs):
> | 
> |       43.367.405,59 msec task-clock                #   30,901 CPUs utilized            ( +-  0,01% )
> |           7.393.238      context-switches          #  170,499 /sec                     ( +-  0,13% )
> |             832.364      cpu-migrations            #   19,196 /sec                     ( +-  0,15% )
> |         625.235.644      page-faults               #   14,419 K/sec                    ( +-  0,00% )
> | 103.822.081.026.160      cycles                    #    2,394 GHz                      ( +-  0,01% )
> |  75.392.684.840.822      stalled-cycles-frontend   #   72,63% frontend cycles idle     ( +-  0,02% )
> |  54.971.177.787.990      stalled-cycles-backend    #   52,95% backend cycles idle      ( +-  0,02% )
> |  69.543.893.308.966      instructions              #    0,67  insn per cycle
> |                                                    #    1,08  stalled cycles per insn  ( +-  0,00% )
> |  14.585.269.354.314      branches                  #  336,357 M/sec                    ( +-  0,00% )
> |     558.029.270.966      branch-misses             #    3,83% of all branches          ( +-  0,01% )
> |  
> |            1403,441 +- 0,466 seconds time elapsed  ( +-  0,03% )
> 
> 
> With the optimisation disabled:
> |  Performance counter stats for './b.sh' (5 runs):
> | 
> |       43.354.742,31 msec task-clock                #   30,869 CPUs utilized            ( +-  0,01% )
> |           7.394.210      context-switches          #  170,601 /sec                     ( +-  0,06% )
> |             842.835      cpu-migrations            #   19,446 /sec                     ( +-  0,63% )
> |         625.242.341      page-faults               #   14,426 K/sec                    ( +-  0,00% )
> | 103.791.714.272.978      cycles                    #    2,395 GHz                      ( +-  0,01% )
> |  75.369.652.256.425      stalled-cycles-frontend   #   72,64% frontend cycles idle     ( +-  0,01% )
> |  54.947.610.706.450      stalled-cycles-backend    #   52,96% backend cycles idle      ( +-  0,01% )
> |  69.529.388.440.691      instructions              #    0,67  insn per cycle
> |                                                    #    1,08  stalled cycles per insn  ( +-  0,01% )
> |  14.584.515.016.870      branches                  #  336,497 M/sec                    ( +-  0,00% )
> |     557.716.885.609      branch-misses             #    3,82% of all branches          ( +-  0,02% )
> |  
> |             1404,47 +- 1,05 seconds time elapsed  ( +-  0,08% )
> 
> I'm still open to a more specific test ;)

Thanks for this test. I do assume that both have been run inside a
non-root memcg.

Weiman, what was the original motivation for 559271146efc0? Because as
this RT patch shows it makes future changes much more complex and I
would prefer a simpler and easier to maintain code than some micro
optimizations that do not have any visible effect on real workloads.
-- 
Michal Hocko
SUSE Labs