Re: [PATCH 1/4] mm/slub: simplify the last resort slab order calculation

[Feng Tang]

On Wed, Sep 20, 2023 at 08:38:05AM +0200, Vlastimil Babka wrote:
> On 9/19/23 09:56, Feng Tang wrote:
> > Hi Vlastimil,
> > 
> > On Fri, Sep 08, 2023 at 10:53:04PM +0800, Vlastimil Babka wrote:
> >> If calculate_order() can't fit even a single large object within
> >> slub_max_order, it will try using the smallest necessary order that may
> >> exceed slub_max_order but not MAX_ORDER.
> >> 
> >> Currently this is done with a call to calc_slab_order() which is
> >> unecessary. We can simply use get_order(size). No functional change.
> >> 
> >> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> >> ---
> >>  mm/slub.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/mm/slub.c b/mm/slub.c
> >> index f7940048138c..c6e694cb17b9 100644
> >> --- a/mm/slub.c
> >> +++ b/mm/slub.c
> >> @@ -4193,7 +4193,7 @@ static inline int calculate_order(unsigned int size)
> >>  	/*
> >>  	 * Doh this slab cannot be placed using slub_max_order.
> >>  	 */
> >> -	order = calc_slab_order(size, 1, MAX_ORDER, 1);
> >> +	order = get_order(size);
> > 
> > 
> > This patchset is a nice cleanup, and my previous test all looked fine. 
> > And one 'slub_min_order' setup reminded by Christopher [1] doesn't
> > work as not taking affect with this 1/4 patch.
> 
> Hmm I see. Well the trick should keep working if you pass both
> slab_min_order=9 slab_max_order=9 ? Maybe Christopher actually does that,
> but didn't type it fully in the mail.

Yes, that's possible. And "slub_min_order=9" alone also works to make
all slab's order be 9, as current code's final fallback will try
MAX_ORDER inside caculate_order():

	order = calc_slab_order(size, 1, MAX_ORDER, 1);

though the dmesg looks strange:

  SLUB: HWalign=64, Order=9-3, MinObjects=0, CPUs=16, Nodes=1

> 
> > The root cause seems to be, in current kernel, the 'slub_max_order'
> > is not ajusted  accordingly with 'slub_min_order', so there is case
> > that 'slub_min_order' is bigger than the default 'slub_max_order' (3).
> > 
> > And it could be fixed by the below patch 
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 1c91f72c7239..dbe950783105 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -4702,6 +4702,9 @@ static int __init setup_slub_min_order(char *str)
> >  {
> >  	get_option(&str, (int *)&slub_min_order);
> >  
> > +	if (slub_min_order > slub_max_order)
> > +		slub_max_order = slub_min_order;
> > +
> >  	return 1;
> >  }
> 
> Sounds like a good idea. Would also do analogous thing in setup_slub_max_order.

Yes.

> > Though the formal fix may also need to cover case like this kind of
> > crazy setting "slub_min_order=6 slub_max_order=5" 
> 
> Doing both should cover even this, and AFAICS how param processing works the
> last one passed would "win" so it would set min=max=5 in that case. That's
> probably the most sane way we can handle such scenarios.

Agree. The latter setting should take privilage. My test code also
does this way.

> Want to set a full patch or should I finalize it? I would put it as a new
> 1/5 before the rest. Thanks!

I can try to make a patch with more detail in commit log, and resend. Thanks
for the review!

Thanks,
Feng

> 
> > [1]. https://lore.kernel.org/lkml/21a0ba8b-bf05-0799-7c78-2a35f8c8d52a@xxxxxxxxxxxxxxxxxxxxxx/
> > 
> > Thanks,
> > Feng
> > 
> >>  	if (order <= MAX_ORDER)
> >>  		return order;
> >>  	return -ENOSYS;
> >> -- 
> >> 2.42.0
> >> 
> >> 
>