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If that's impossible (for particular large object > sizes), there's a fallback that will try to place one object within > slab_max_order. > > If we expand the loop boundary to also allow up to 1/2 waste as the last > resort, we can remove the fallback and simplify the code, as the loop > will find an order for such sizes as well. Note we don't need to allow > more than 1/2 waste as that will never happen - calc_slab_order() would > calculate more objects to fit, reducing waste below 1/2. > > Sucessfully finding an order in the loop (compared to the fallback) will > also have the benefit in trying to satisfy min_objects, because the > fallback was passing 1. Thus the resulting slab orders might be larger > (not because it would improve waste, but to reduce pressure on shared > locks), which is one of the goals of calculate_order(). > > For example, with nr_cpus=1 and 4kB PAGE_SIZE, slub_max_order=3, before > the patch we would get the following orders for these object sizes: > > 2056 to 10920 - order-3 as selected by the loop > 10928 to 12280 - order-2 due to fallback, as <1/4 waste is not possible > 12288 to 32768 - order-3 as <1/4 waste is again possible > > After the patch: > > 2056 to 32768 - order-3, because even in the range of 10928 to 12280 we > try to satisfy the calculated min_objects. > > As a result the code is simpler and gives more consistent results. Current code already tries the fraction "1" in the follwing 2 fallback calls of calc_slab_order(), so trying fraction "2" makes sense to me. Reviewed-by: Feng Tang Thanks, Feng > Signed-off-by: Vlastimil Babka > --- > mm/slub.c | 14 ++++---------- > 1 file changed, 4 insertions(+), 10 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 5c287d96b212..f04eb029d85a 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -4171,23 +4171,17 @@ static inline int calculate_order(unsigned int size) > * the order can only result in same or less fractional waste, not more. > * > * If that fails, we increase the acceptable fraction of waste and try > - * again. > + * again. The last iteration with fraction of 1/2 would effectively > + * accept any waste and give us the order determined by min_objects, as > + * long as at least single object fits within slub_max_order. > */ > - for (unsigned int fraction = 16; fraction >= 4; fraction /= 2) { > + for (unsigned int fraction = 16; fraction > 1; fraction /= 2) { > order = calc_slab_order(size, min_objects, slub_max_order, > fraction); > if (order <= slub_max_order) > return order; > } > > - /* > - * We were unable to place multiple objects in a slab. Now > - * lets see if we can place a single object there. > - */ > - order = calc_slab_order(size, 1, slub_max_order, 1); > - if (order <= slub_max_order) > - return order; > - > /* > * Doh this slab cannot be placed using slub_max_order. > */ > -- > 2.42.0 > >