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Subject: Re: [PATCH v2 2/6] writeback: collect stats of all wb of bdi in
 bdi_debug_stats_show
To: Brian Foster <bfoster@redhat.com>
Cc: akpm@linux-foundation.org, willy@infradead.org, jack@suse.cz,
 tj@kernel.org, dsterba@suse.com, mjguzik@gmail.com, dhowells@redhat.com,
 linux-kernel@vger.kernel.org, linux-mm@kvack.org,
 linux-fsdevel@vger.kernel.org
References: <20240327155751.3536-1-shikemeng@huaweicloud.com>
 <20240327155751.3536-3-shikemeng@huaweicloud.com> <Zga8Sf1DIxMevdcw@bfoster>
From: Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <9a9fe07e-d7f4-19f7-a6fb-28ae0ca4c25e@huaweicloud.com>
Date: Wed, 3 Apr 2024 15:49:07 +0800
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on 3/29/2024 9:04 PM, Brian Foster wrote:
> On Wed, Mar 27, 2024 at 11:57:47PM +0800, Kemeng Shi wrote:
>> /sys/kernel/debug/bdi/xxx/stats is supposed to show writeback information
>> of whole bdi, but only writeback information of bdi in root cgroup is
>> collected. So writeback information in non-root cgroup are missing now.
>> To be more specific, considering following case:
>>
>> /* create writeback cgroup */
>> cd /sys/fs/cgroup
>> echo "+memory +io" > cgroup.subtree_control
>> mkdir group1
>> cd group1
>> echo $$ > cgroup.procs
>> /* do writeback in cgroup */
>> fio -name test -filename=/dev/vdb ...
>> /* get writeback info of bdi */
>> cat /sys/kernel/debug/bdi/xxx/stats
>> The cat result unexpectedly implies that there is no writeback on target
>> bdi.
>>
>> Fix this by collecting stats of all wb in bdi instead of only wb in
>> root cgroup.
>>
>> Following domain hierarchy is tested:
>>                 global domain (320G)
>>                 /                 \
>>         cgroup domain1(10G)     cgroup domain2(10G)
>>                 |                 |
>> bdi            wb1               wb2
>>
>> /* all writeback info of bdi is successfully collected */
>> cat stats
>> BdiWriteback:             2912 kB
>> BdiReclaimable:        1598464 kB
>> BdiDirtyThresh:      167479028 kB
>> DirtyThresh:         195038532 kB
>> BackgroundThresh:     32466728 kB
>> BdiDirtied:           19141696 kB
>> BdiWritten:           17543456 kB
>> BdiWriteBandwidth:     1136172 kBps
>> b_dirty:                     2
>> b_io:                        0
>> b_more_io:                   1
>> b_dirty_time:                0
>> bdi_list:                    1
>> state:                       1
>>
>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>> ---
>>  mm/backing-dev.c | 100 +++++++++++++++++++++++++++++++++--------------
>>  1 file changed, 71 insertions(+), 29 deletions(-)
>>
>> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
>> index 70f02959f3bd..8daf950e6855 100644
>> --- a/mm/backing-dev.c
>> +++ b/mm/backing-dev.c
> ...
>> @@ -65,16 +78,54 @@ static struct backing_dev_info *lookup_bdi(struct seq_file *m)
>>  	return NULL;
>>  }
>>  
>> +static void collect_wb_stats(struct wb_stats *stats,
>> +			     struct bdi_writeback *wb)
>> +{
>> +	struct inode *inode;
>> +
>> +	spin_lock(&wb->list_lock);
>> +	list_for_each_entry(inode, &wb->b_dirty, i_io_list)
>> +		stats->nr_dirty++;
>> +	list_for_each_entry(inode, &wb->b_io, i_io_list)
>> +		stats->nr_io++;
>> +	list_for_each_entry(inode, &wb->b_more_io, i_io_list)
>> +		stats->nr_more_io++;
>> +	list_for_each_entry(inode, &wb->b_dirty_time, i_io_list)
>> +		if (inode->i_state & I_DIRTY_TIME)
>> +			stats->nr_dirty_time++;
>> +	spin_unlock(&wb->list_lock);
>> +
>> +	stats->nr_writeback += wb_stat(wb, WB_WRITEBACK);
>> +	stats->nr_reclaimable += wb_stat(wb, WB_RECLAIMABLE);
>> +	stats->nr_dirtied += wb_stat(wb, WB_DIRTIED);
>> +	stats->nr_written += wb_stat(wb, WB_WRITTEN);
>> +	stats->wb_thresh += wb_calc_thresh(wb, stats->dirty_thresh);
> 
> Kinda nitty question, but is this a sum of per-wb writeback thresholds?
> If so, do you consider that useful information vs. the per-wb threshold
> data presumably exposed in the next patch?
It's sum of per-wb wirteback thresholds in global domain. For each wb,
it's threshold is min of threshold in global domain and threshold in
cgroup domain (if any). As the debug data of bdi existed before writeback
cgroup was introduced, so it would be better to show bdi threshold in global
domain which is more compatible.
> 
> I'm not really that worried about what debug data we expose, it just
> seems a little odd. How would you document this value in a sentence or
> two, for example?
I think it could simply be "threshold of bdi in global domain".
> 
>> +}
>> +
>> +#ifdef CONFIG_CGROUP_WRITEBACK
>> +static void bdi_collect_stats(struct backing_dev_info *bdi,
>> +			      struct wb_stats *stats)
>> +{
>> +	struct bdi_writeback *wb;
>> +
>> +	list_for_each_entry_rcu(wb, &bdi->wb_list, bdi_node)
>> +		collect_wb_stats(stats, wb);
> 
> Depending on discussion on the previous patch and whether the higher
> level rcu protection in bdi_debug_stats_show() is really necessary, it
> might make more sense to move it here.
Sure, will do it in next version.
> 
> I'm also wondering if you'd want to check the state of the individual wb
> (i.e. WB_registered?) before reading it..?
I think it't better to keep full debug info. The user could filter it out
with state in debug info anyway.
> 
>> +}
>> +#else
>> +static void bdi_collect_stats(struct backing_dev_info *bdi,
>> +			      struct wb_stats *stats)
>> +{
>> +	collect_wb_stats(stats, &bdi->wb);
>> +}
>> +#endif
> ...
>> @@ -115,18 +157,18 @@ static int bdi_debug_stats_show(struct seq_file *m, void *v)
>>  		   "b_dirty_time:       %10lu\n"
>>  		   "bdi_list:           %10u\n"
>>  		   "state:              %10lx\n",
>> -		   (unsigned long) K(wb_stat(wb, WB_WRITEBACK)),
>> -		   (unsigned long) K(wb_stat(wb, WB_RECLAIMABLE)),
>> -		   K(wb_thresh),
>> +		   K(stats.nr_writeback),
>> +		   K(stats.nr_reclaimable),
>> +		   K(stats.wb_thresh),
>>  		   K(dirty_thresh),
>>  		   K(background_thresh),
>> -		   (unsigned long) K(wb_stat(wb, WB_DIRTIED)),
>> -		   (unsigned long) K(wb_stat(wb, WB_WRITTEN)),
>> -		   (unsigned long) K(wb->write_bandwidth),
>> -		   nr_dirty,
>> -		   nr_io,
>> -		   nr_more_io,
>> -		   nr_dirty_time,
>> +		   K(stats.nr_dirtied),
>> +		   K(stats.nr_written),
>> +		   K(tot_bw),
>> +		   stats.nr_dirty,
>> +		   stats.nr_io,
>> +		   stats.nr_more_io,
>> +		   stats.nr_dirty_time,
>>  		   !list_empty(&bdi->bdi_list), bdi->wb.state);
> 
> Is it worth showing a list count here rather than list_empty() state?
Actually, I don't know how this info was supposed to be used. So I keep it in
old way for compatibility...
As for bdi count, it would be easy to retrieve by counting the bdi number under
/sys/kernel/debug/bdi/.
As for wb count, it would be easy to count with wb_stats.
So I still prefer to keep it in old way for compatibility or just simply remove
it if the list_empty() state is not needed.
Thansk for the review and all advise. Look forward to your reply.

Kemeng

> 
> Brian
> 
>>  
>>  	rcu_read_unlock();
>> -- 
>> 2.30.0
>>
>