Re: [PATCH mm-unstable] lib/Kconfig.debug: do not enable DEBUG_PREEMPT by default

[Michal Hocko]

On Fri 27-01-23 20:43:20, Hyeonggon Yoo wrote:
> On Wed, Jan 25, 2023 at 10:51:05AM +0100, Michal Hocko wrote:
> > On Thu 26-01-23 00:41:15, Hyeonggon Yoo wrote:
> > [...]
> > > > Do you happen to have any perf data collected during those runs? I
> > > > would be interested in the memcg side of things. Maybe we can do
> > > > something better there.
> > > 
> > > Yes, below is performance data I've collected.
> > > 
> > > 6.1.8-debug-preempt-dirty
> > > =========================
> > >   Overhead  Command       Shared Object     Symbol
> > > +    9.14%  hackbench        [kernel.vmlinux]  [k] check_preemption_disabled
> > 
> > Thanks! Could you just add callers that are showing in the profile for
> > this call please?
> 
> -   14.56%     9.14%  hackbench        [kernel.vmlinux]  [k] check_preemption_disabled                          
>    - 6.37% check_preemption_disabled                                                                            
>       + 3.48% mod_objcg_state                                                                                   
>       + 1.10% obj_cgroup_charge                                                                                 
>         1.02% refill_obj_stock                                                                                  
>      0.67% memcg_slab_post_alloc_hook                                                                           
>      0.58% mod_objcg_state      
> 
> According to perf, many memcg functions call this function
> and that's because __this_cpu_xxxx checks if preemption is disabled.

OK, I see. Thanks! I was thinking whether we can optimize for that bu
IIUC __this_cpu* is already an optimized form. mod_objcg_state is
already called with local_lock so raw_cpu* could be used in that path
but I guess this is not really worth just to optimize for a debug
compile option to benefit.
-- 
Michal Hocko
SUSE Labs