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Date:   Thu, 17 May 2018 07:49:24 +0300
From:   Vladimir Davydov <vdavydov.dev@gmail.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     akpm@linux-foundation.org, shakeelb@google.com,
        viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
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Subject: Re: [PATCH v5 13/13] mm: Clear shrinker bit if there are no objects
 related to memcg
Message-ID: <20180517044924.5tq6vbqituvr3nzh@esperanza>
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On Tue, May 15, 2018 at 11:55:04AM +0300, Kirill Tkhai wrote:
> >> @@ -586,8 +586,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
> >>  			continue;
> >>  
> >>  		ret = do_shrink_slab(&sc, shrinker, priority);
> >> -		if (ret == SHRINK_EMPTY)
> >> -			ret = 0;
> >> +		if (ret == SHRINK_EMPTY) {
> >> +			clear_bit(i, map->map);
> >> +			/*
> >> +			 * Pairs with mb in memcg_set_shrinker_bit():
> >> +			 *
> >> +			 * list_lru_add()     shrink_slab_memcg()
> >> +			 *   list_add_tail()    clear_bit()
> >> +			 *   <MB>               <MB>
> >> +			 *   set_bit()          do_shrink_slab()
> >> +			 */
> > 
> > Please improve the comment so that it isn't just a diagram.
> 
> Please, say, which comment you want to see here.

I want the reader to understand why we need to invoke the shrinker twice
if it returns SHRINK_EMPTY. The diagram doesn't really help here IMO. So
I'd write Something like this:

	ret = do_shrink_slab(&sc, shrinker, priority);
	if (ret == SHRINK_EMPTY) {
		clear_bit(i, map->map);
		/*
		 * After the shrinker reported that it had no objects to free,
		 * but before we cleared the corresponding bit in the memcg
		 * shrinker map, a new object might have been added. To make
		 * sure, we have the bit set in this case, we invoke the
		 * shrinker one more time and re-set the bit if it reports that
		 * it is not empty anymore. The memory barrier here pairs with
		 * the barrier in memcg_set_shrinker_bit():
		 *
		 * list_lru_add()     shrink_slab_memcg()
		 *   list_add_tail()    clear_bit()
		 *   <MB>               <MB>
		 *   set_bit()          do_shrink_slab()
		 */
		smp_mb__after_atomic();
		ret = do_shrink_slab(&sc, shrinker, priority);
			if (ret == SHRINK_EMPTY)
				ret = 0;
			else
				memcg_set_shrinker_bit(memcg, nid, i);