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Subject: Re: [PATCH 1/6] writeback: collect stats of all wb of bdi in
 bdi_debug_stats_show
To: Brian Foster <bfoster@redhat.com>
Cc: akpm@linux-foundation.org, tj@kernel.org, linux-mm@kvack.org,
 linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
 willy@infradead.org, jack@suse.cz, dsterba@suse.com, mjguzik@gmail.com,
 dhowells@redhat.com, peterz@infradead.org
References: <20240320110222.6564-1-shikemeng@huaweicloud.com>
 <20240320110222.6564-2-shikemeng@huaweicloud.com> <Zfriwb03HCRWJ24q@bfoster>
From: Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <3d08c249-1b12-f82b-2662-a6fa2b888011@huaweicloud.com>
Date: Thu, 21 Mar 2024 11:44:40 +0800
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on 3/20/2024 9:21 PM, Brian Foster wrote:
> On Wed, Mar 20, 2024 at 07:02:17PM +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.
>>
>> Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
>> ---
>>  mm/backing-dev.c | 93 ++++++++++++++++++++++++++++++++++++------------
>>  1 file changed, 70 insertions(+), 23 deletions(-)
>>
>> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
>> index 5f2be8c8df11..788702b6c5dd 100644
>> --- a/mm/backing-dev.c
>> +++ b/mm/backing-dev.c
> ...
>> @@ -46,31 +59,65 @@ static void bdi_debug_init(void)
>>  	bdi_debug_root = debugfs_create_dir("bdi", NULL);
>>  }
>>  
> ...
>> +#ifdef CONFIG_CGROUP_WRITEBACK
>> +static void bdi_collect_stats(struct backing_dev_info *bdi,
>> +			      struct wb_stats *stats)
>> +{
>> +	struct bdi_writeback *wb;
>> +
>> +	/* protect wb from release */
>> +	mutex_lock(&bdi->cgwb_release_mutex);
>> +	list_for_each_entry(wb, &bdi->wb_list, bdi_node)
>> +		collect_wb_stats(stats, wb);
>> +	mutex_unlock(&bdi->cgwb_release_mutex);
>> +}
>> +#else
>> +static void bdi_collect_stats(struct backing_dev_info *bdi,
>> +			      struct wb_stats *stats)
>> +{
>> +	collect_wb_stats(stats, &bdi->wb);
>> +}
>> +#endif
>> +
> 
> I'm not familiar enough with the cgwb code to say for sure (and I'd
> probably wait for more high level feedback before worrying too much
> about this), but do we need the ifdef here just to iterate ->wb_list?
>>From looking at the code, it appears bdi->wb ends up on the list while
> the bdi is registered for both cases, so that distinction seems
> unnecessary. WRT to wb release protection, I wonder if this could use a
Currently, we have ifdef trying to remove unnecessary cost when
CONFIG_CGROUP_WRITEBACK is not enabled, see defination of cgwb_bdi_register
and cgwb_remove_from_bdi_list for example. So I try to define bdi_collect_stats
in similar way.
> combination of rcu_read_lock()/list_for_each_safe() and wb_tryget() on
> each wb before collecting its stats..? See how bdi_split_work_to_wbs()
> works, for example.
The combination of rcu_read_lock()/list_for_each_safe() and wb_tryget()
should work fine.
With ifdef, bdi_collect_stats takes no extra cost when CONFIG_CGROUP_WRITEBACK
is not enabled and is consistent with existing code style, so I still prefer
this way. Yes, The extra cost is not a big deal as it only exists in debug mode,
so it's acceptable to use the suggested combination in next version if you are
still strongly aganst this.

> 
> Also I see a patch conflict/compile error on patch 2 due to
> __wb_calc_thresh() only taking one parameter in my tree. What's the
> baseline commit for this series?
> 
Sorry for missing this, this seris is based on another patchset [1] which is still
under review.
Look forward to your reply!

Thansk
Kemeng

[1] https://lore.kernel.org/lkml/20240123183332.876854-1-shikemeng@huaweicloud.com/T/#mc6455784a63d0f8aa1a2f5aff325abcdf9336b76

> Brian
> 
>> +static int bdi_debug_stats_show(struct seq_file *m, void *v)
>> +{
>> +	struct backing_dev_info *bdi = m->private;
>> +	unsigned long background_thresh;
>> +	unsigned long dirty_thresh;
>> +	struct wb_stats stats;
>> +	unsigned long tot_bw;
>> +
>>  	global_dirty_limits(&background_thresh, &dirty_thresh);
>> -	wb_thresh = wb_calc_thresh(wb, dirty_thresh);
>> +
>> +	memset(&stats, 0, sizeof(stats));
>> +	stats.dirty_thresh = dirty_thresh;
>> +	bdi_collect_stats(bdi, &stats);
>> +
>> +	tot_bw = atomic_long_read(&bdi->tot_write_bandwidth);
>>  
>>  	seq_printf(m,
>>  		   "BdiWriteback:       %10lu kB\n"
>> @@ -87,18 +134,18 @@ 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);
>>  
>>  	return 0;
>> -- 
>> 2.30.0
>>
>>
> 
>