Subject: Re: [PATCH 12/13] mm: memcontrol: account socket memory in unified hierarchy memory controller

From: Johannes Weiner <[email-masked]>
Date: Mon, 30 Nov 2015 10:26:38 -0500

On Mon, Nov 30, 2015 at 01:54:21PM +0300, Vladimir Davydov wrote:
> On Tue, Nov 24, 2015 at 04:58:44PM -0500, Johannes Weiner wrote:
> ...
> > @@ -5520,15 +5557,30 @@ void sock_release_memcg(struct sock *sk)
> >   */
> >  bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
> >  {
> > -	struct page_counter *counter;
> > +	gfp_t gfp_mask = GFP_KERNEL;
> >  
> > -	if (page_counter_try_charge(&memcg->tcp_mem.memory_allocated,
> > -				    nr_pages, &counter)) {
> > -		memcg->tcp_mem.memory_pressure = 0;
> > -		return true;
> > +#ifdef CONFIG_MEMCG_KMEM
> > +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> > +		struct page_counter *counter;
> > +
> > +		if (page_counter_try_charge(&memcg->tcp_mem.memory_allocated,
> > +					    nr_pages, &counter)) {
> > +			memcg->tcp_mem.memory_pressure = 0;
> > +			return true;
> > +		}
> > +		page_counter_charge(&memcg->tcp_mem.memory_allocated, nr_pages);
> > +		memcg->tcp_mem.memory_pressure = 1;
> > +		return false;
> >  	}
> > -	page_counter_charge(&memcg->tcp_mem.memory_allocated, nr_pages);
> > -	memcg->tcp_mem.memory_pressure = 1;
> > +#endif
> > +	/* Don't block in the packet receive path */
> > +	if (in_softirq())
> > +		gfp_mask = GFP_NOWAIT;
> > +
> > +	if (try_charge(memcg, gfp_mask, nr_pages) == 0)
> > +		return true;
> > +
> > +	try_charge(memcg, gfp_mask|__GFP_NOFAIL, nr_pages);
> 
> We won't trigger high reclaim if we get here, because try_charge does
> not check high threshold if failing or forcing charge. I think this
> should be fixed regardless of this patch. The fix is attached below.

We kind of assume that max is either set above high, or not at
all. That means when max is hit the high limit has already failed and
it's of limited use to schedule background reclaim.

> Also, I don't like calling try_charge twice: the second time will go
> through all the try_charge steps for nothing. What about checking
> page_counter value after calling try_charge instead:
> 
> 	try_charge(memcg, gfp_mask|__GFP_NOFAIL, nr_pages);
> 	return page_counter_read(&memcg->memory) <= memcg->memory.limit;
> 
> or adding an out parameter to try_charge that would inform us if charge
> was forced?

That's a complete cold path where we are going to drop the packet in
all but a few cases. It's not worth the trouble.

> > @@ -5539,10 +5591,32 @@ bool mem_cgroup_charge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
> >   */
> >  void mem_cgroup_uncharge_skmem(struct mem_cgroup *memcg, unsigned int nr_pages)
> >  {
> > -	page_counter_uncharge(&memcg->tcp_mem.memory_allocated, nr_pages);
> > +#ifdef CONFIG_MEMCG_KMEM
> > +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> > +		page_counter_uncharge(&memcg->tcp_mem.memory_allocated,
> > +				      nr_pages);
> > +		return;
> > +	}
> > +#endif
> > +	page_counter_uncharge(&memcg->memory, nr_pages);
> > +	css_put_many(&memcg->css, nr_pages);
> 
> cancel_charge(memcg, nr_pages);

It does the same, but it's a weird name for regular uncharging.

> From: Vladimir Davydov <[email-masked]>
> Subject: [PATCH] memcg: check high threshold if forcing allocation
> 
> try_charge() does not result in checking high threshold if it forces
> charge. This is incorrect, because we could have failed to reclaim
> memory due to the current context, so we do need to check high threshold
> and try to compensate for the excess once we are in the safe context.
> 
> Signed-off-by: Vladimir Davydov <[email-masked]>
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 79a29d564bff..e922965b572b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2112,13 +2112,14 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  		page_counter_charge(&memcg->memsw, nr_pages);
>  	css_get_many(&memcg->css, nr_pages);
>  
> -	return 0;
> +	goto check_high;
>  
>  done_restock:
>  	css_get_many(&memcg->css, batch);
>  	if (batch > nr_pages)
>  		refill_stock(memcg, batch - nr_pages);
>  
> +check_high:
>  	/*
>  	 * If the hierarchy is above the normal consumption range, schedule
>  	 * reclaim on returning to userland.  We can perform reclaim here

One problem is that OOM victims force their charges so they can exit
quickly. It'd be contradictory to then task them with high reclaim.