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Date:   Fri, 29 Mar 2019 10:50:17 -0700
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Subject: Re: [PATCH] writeback: sum memcg dirty counters as needed
From:   Greg Thelen <gthelen@google.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Tejun Heo <tj@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Thu, Mar 07, 2019 at 08:56:32AM -0800, Greg Thelen wrote:
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3880,6 +3880,7 @@ struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
>>   * @pheadroom: out parameter for number of allocatable pages according to memcg
>>   * @pdirty: out parameter for number of dirty pages
>>   * @pwriteback: out parameter for number of pages under writeback
>> + * @exact: determines exact counters are required, indicates more work.
>>   *
>>   * Determine the numbers of file, headroom, dirty, and writeback pages in
>>   * @wb's memcg.  File, dirty and writeback are self-explanatory.  Headroom
>> @@ -3890,18 +3891,29 @@ struct wb_domain *mem_cgroup_wb_domain(struct bdi_writeback *wb)
>>   * ancestors.  Note that this doesn't consider the actual amount of
>>   * available memory in the system.  The caller should further cap
>>   * *@pheadroom accordingly.
>> + *
>> + * Return value is the error precision associated with *@pdirty
>> + * and *@pwriteback.  When @exact is set this a minimal value.
>>   */
>> -void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>> -			 unsigned long *pheadroom, unsigned long *pdirty,
>> -			 unsigned long *pwriteback)
>> +unsigned long
>> +mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>> +		    unsigned long *pheadroom, unsigned long *pdirty,
>> +		    unsigned long *pwriteback, bool exact)
>>  {
>>  	struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css);
>>  	struct mem_cgroup *parent;
>> +	unsigned long precision;
>>  
>> -	*pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
>> -
>> +	if (exact) {
>> +		precision = 0;
>> +		*pdirty = memcg_exact_page_state(memcg, NR_FILE_DIRTY);
>> +		*pwriteback = memcg_exact_page_state(memcg, NR_WRITEBACK);
>> +	} else {
>> +		precision = MEMCG_CHARGE_BATCH * num_online_cpus();
>> +		*pdirty = memcg_page_state(memcg, NR_FILE_DIRTY);
>> +		*pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
>> +	}
>>  	/* this should eventually include NR_UNSTABLE_NFS */
>> -	*pwriteback = memcg_page_state(memcg, NR_WRITEBACK);
>>  	*pfilepages = mem_cgroup_nr_lru_pages(memcg, (1 << LRU_INACTIVE_FILE) |
>>  						     (1 << LRU_ACTIVE_FILE));
>>  	*pheadroom = PAGE_COUNTER_MAX;
>> @@ -3913,6 +3925,8 @@ void mem_cgroup_wb_stats(struct bdi_writeback *wb, unsigned long *pfilepages,
>>  		*pheadroom = min(*pheadroom, ceiling - min(ceiling, used));
>>  		memcg = parent;
>>  	}
>> +
>> +	return precision;
>
> Have you considered unconditionally using the exact version here?
>
> It does for_each_online_cpu(), but until very, very recently we did
> this per default for all stats, for years. It only became a problem in
> conjunction with the for_each_memcg loops when frequently reading
> memory stats at the top of a very large hierarchy.
>
> balance_dirty_pages() is called against memcgs that actually own the
> inodes/memory and doesn't do the additional recursive tree collection.
>
> It's also not *that* hot of a function, and in the io path...
>
> It would simplify this patch immensely.

Good idea.  Done in -v2 of the patch:
https://lore.kernel.org/lkml/20190329174609.164344-1-gthelen@google.com/