linux-kernel - Re: [PATCH v3 03/19] mm: memcg: convert vmstat slab counters to bytes

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Date:   Wed, 20 May 2020 12:26:52 -0700
From:   Roman Gushchin <guro@...com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 03/19] mm: memcg: convert vmstat slab counters to bytes

On Wed, May 20, 2020 at 02:25:22PM +0200, Vlastimil Babka wrote:
> On 4/22/20 10:46 PM, Roman Gushchin wrote:
> > In order to prepare for per-object slab memory accounting, convert
> > NR_SLAB_RECLAIMABLE and NR_SLAB_UNRECLAIMABLE vmstat items to bytes.
> > 
> > To make it obvious, rename them to NR_SLAB_RECLAIMABLE_B and
> > NR_SLAB_UNRECLAIMABLE_B (similar to NR_KERNEL_STACK_KB).
> > 
> > Internally global and per-node counters are stored in pages,
> > however memcg and lruvec counters are stored in bytes.
> > This scheme may look weird, but only for now. As soon as slab
> > pages will be shared between multiple cgroups, global and
> > node counters will reflect the total number of slab pages.
> > However memcg and lruvec counters will be used for per-memcg
> > slab memory tracking, which will take separate kernel objects
> > in the account. Keeping global and node counters in pages helps
> > to avoid additional overhead.
> > 
> > The size of slab memory shouldn't exceed 4Gb on 32-bit machines,
> > so it will fit into atomic_long_t we use for vmstats.
> > 
> > Signed-off-by: Roman Gushchin <guro@...com>
> > ---
> >  drivers/base/node.c     |  4 ++--
> >  fs/proc/meminfo.c       |  4 ++--
> >  include/linux/mmzone.h  | 16 +++++++++++++---
> >  kernel/power/snapshot.c |  2 +-
> >  mm/memcontrol.c         | 11 ++++-------
> >  mm/oom_kill.c           |  2 +-
> >  mm/page_alloc.c         |  8 ++++----
> >  mm/slab.h               | 15 ++++++++-------
> >  mm/slab_common.c        |  4 ++--
> >  mm/slob.c               | 12 ++++++------
> >  mm/slub.c               |  8 ++++----
> >  mm/vmscan.c             |  3 ++-
> >  mm/workingset.c         |  6 ++++--
> >  13 files changed, 53 insertions(+), 42 deletions(-)
> 
> 
> > @@ -206,7 +206,17 @@ enum node_stat_item {
> >  
> >  static __always_inline bool vmstat_item_in_bytes(enum node_stat_item item)
> >  {
> > -	return false;
> > +	/*
> > +	 * Global and per-node slab counters track slab pages.
> > +	 * It's expected that changes are multiples of PAGE_SIZE.
> > +	 * Internally values are stored in pages.
> > +	 *
> > +	 * Per-memcg and per-lruvec counters track memory, consumed
> > +	 * by individual slab objects. These counters are actually
> > +	 * byte-precise.
> > +	 */
> > +	return (item == NR_SLAB_RECLAIMABLE_B ||
> > +		item == NR_SLAB_UNRECLAIMABLE_B);

Hello, Vlastimil!

Thank you for looking into the patchset, appreciate it.
In the next version I'll add some comments based on your suggestions in previous
letters.

> >  }
> 
> Ok, so this is no longer a no-op, but __always_inline here and inline in
> global_node_page_state() should hopefully mean that for all users of
> global_node_page_state(<constant>) the compiler will eliminate the branch for
> non-slab counters. But there are also functions such as si_mem_available() that
> use non-constant item. Maybe compiler is smart enough anyway, but perhaps it's
> better to use global_node_page_state_pages() in such callers?

I'll take a look, thanks for the idea.

> 
> However __mod_node_page_state() and mode_node_state() will now branch always. I
> wonder if the "API clean" goal is worth it...

You mean just adding a special write-side helper which will perform a conversion
and put VM_WARN_ON_ONCE() into generic write-side helpers?

> 
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1409,9 +1409,8 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
> >  		       (u64)memcg_page_state(memcg, MEMCG_KERNEL_STACK_KB) *
> >  		       1024);
> >  	seq_buf_printf(&s, "slab %llu\n",
> > -		       (u64)(memcg_page_state(memcg, NR_SLAB_RECLAIMABLE) +
> > -			     memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE)) *
> > -		       PAGE_SIZE);
> > +		       (u64)(memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B) +
> > +			     memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B)));
> >  	seq_buf_printf(&s, "sock %llu\n",
> >  		       (u64)memcg_page_state(memcg, MEMCG_SOCK) *
> >  		       PAGE_SIZE);
> > @@ -1445,11 +1444,9 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
> >  			       PAGE_SIZE);
> >  
> >  	seq_buf_printf(&s, "slab_reclaimable %llu\n",
> > -		       (u64)memcg_page_state(memcg, NR_SLAB_RECLAIMABLE) *
> > -		       PAGE_SIZE);
> > +		       (u64)memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B));
> >  	seq_buf_printf(&s, "slab_unreclaimable %llu\n",
> > -		       (u64)memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE) *
> > -		       PAGE_SIZE);
> > +		       (u64)memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B));
> 
> So here we are now printing in bytes instead of pages, right? That's fine for
> OOM report, but in sysfs aren't we breaking existing users?
> 

Hm, but it was in bytes previously, look at that x * PAGE_SIZE.
Or do you mean that now it can be not rounded to PAGE_SIZE?
If so, I don't think it breaks any expectations.

Thanks!