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

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Date:   Thu, 21 May 2020 11:57:12 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>
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 5/20/20 9:26 PM, Roman Gushchin wrote:
> On Wed, May 20, 2020 at 02:25:22PM +0200, Vlastimil Babka wrote:
>> 
>> 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?

What I mean is that maybe node/global helpers should assume page granularity,
and lruvec/memcg helpers do the check is they should convert from bytes to pages
when calling node/global helpers. Then there would be no extra branches in
node/global helpers. But maybe it's not worth saving those branches, dunno.

>> 
>> > --- 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.

Yeah, that's what I managed to overlook, sorry.

> 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!
>