Re: [PATCH v2] mm/memcontrol: make the slab calculation consistent

[Roman Gushchin <guro@fb.com>]

On Fri, Dec 04, 2020 at 10:46:13AM -0500, Johannes Weiner wrote:
> On Thu, Dec 03, 2020 at 11:11:11AM +0800, Muchun Song wrote:
> > Although the ratio of the slab is one, we also should read the ratio
> > from the related memory_stats instead of hard-coding. And the local
> > variable of size is already the value of slab_unreclaimable. So we
> > do not need to read again. Simplify the code here.
> > 
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > Acked-by: Roman Gushchin <guro@fb.com>
> 
> I agree that ignoring the ratio right now is not very pretty, but
> 
> 		size = memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B) +
> 		       memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B);
> 		seq_buf_printf(&s, "slab %llu\n", size);
> 
> is way easier to understand and more robust than using idx and idx + 1
> and then requiring a series of BUG_ONs to ensure these two items are
> actually adjacent and in the right order.
> 
> There is a redundant call to memcg_page_state(), granted, but that
> function is extremely cheap compared with e.g. seq_buf_printf().
> 
> >  mm/memcontrol.c | 26 +++++++++++++++++++++-----
> >  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> IMO this really just complicates the code with little discernible
> upside. It's going to be a NAK from me, unfortunately.
> 
> 
> In retrospect, I think that memory_stats[] table was a mistake. It
> would probably be easier to implement this using a wrapper for
> memcg_page_state() that has a big switch() for unit
> conversion. Something like this:

+1

> 
> /* Translate stat items to the correct unit for memory.stat output */
> static unsigned long memcg_page_state_output(memcg, item)
> {
> 	unsigned long value = memcg_page_state(memcg, item);
> 	int unit = PAGE_SIZE;
> 
> 	switch (item) {
> 	case NR_SLAB_RECLAIMABLE_B:
> 	case NR_SLAB_UNRECLAIMABLE_B:
> 	case WORKINGSET_REFAULT_ANON:
> 	case WORKINGSET_REFAULT_FILE:
> 	case WORKINGSET_ACTIVATE_ANON:
> 	case WORKINGSET_ACTIVATE_FILE:
> 	case WORKINGSET_RESTORE_ANON:
> 	case WORKINGSET_RESTORE_FILE:
> 	case MEMCG_PERCPU_B:
> 		unit = 1;
> 		break;
> 	case NR_SHMEM_THPS:
> 	case NR_FILE_THPS:
> 	case NR_ANON_THPS:
> 		unit = HPAGE_PMD_SIZE;
> 		break;
             ^^^^^^^^^^^^
These can be easily converted to ordinary pages,
so we can completely avoid this exception.

> 	case NR_KERNEL_STACK_KB:
> 		unit = 1024;
> 		break;
> 	}

And NR_KERNEL_STACK_KB can be converted to bytes.

Then we'll have everything kernel-related in bytes and
everything userspace-related in PAGE_SIZE's.

> 	
> 	return value * unit;
> }
> 
> This would fix the ratio inconsistency, get rid of the awkward mix of
> static and runtime initialization of the table, is probably about the
> same amount of code, but simpler and more obvious overall.

+1