Message-ID: <1caa3057-96e2-41ab-8be1-8e831ecf6afe@vivo.com>
Date: Sat, 11 May 2024 19:05:23 +0800
From: YangYang <yang.yang@...o.com>
To: Mike Snitzer <snitzer@...nel.org>
Cc: Alasdair Kergon <agk@...hat.com>, Mikulas Patocka <mpatocka@...hat.com>,
 dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] dm: Avoid sending redundant empty flush bios to the
 same block device

On 2024/5/9 22:20, Mike Snitzer wrote:
> On Mon, Apr 22, 2024 at 06:05:40PM +0800, Yang Yang wrote:
>> __send_empty_flush() sends empty flush bios to every target in the
>> dm_table. However, if the num_targets exceeds the number of block
>> devices in the dm_table's device list, it could lead to multiple
>> invocations of __send_duplicate_bios() for the same block device.
>> Typically, a single thread sending numerous empty flush bios to one
>> block device is redundant, as these bios are likely to be merged by the
>> flush state machine. In scenarios where num_targets significantly
>> outweighs the number of block devices, such behavior may result in a
>> noteworthy decrease in performance.
>>
>> This issue can be reproduced using this command line:
>>    for i in {0..1023}; do
>>      echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
>>    done | dmsetup create example
> 
> Only _one_ dm_dev will be created for this example: /dev/sda2
> So your bitmap is looking at a single bit.

Yes. And this patch has also been tested on a scenario with multiple
dm_devs using the following command line:
   for i in {0..1023}; do
     if [[ $i -gt 511 ]]; then
       echo $((8000*$i)) 8000 linear /dev/sda3 $((16384*$i))
     else
       echo $((8000*$i)) 8000 linear /dev/sda2 $((16384*$i))
     fi
   done | sudo dmsetup create example

>> With this fix, a random write with fsync workload executed with the
>> following fio command:
>>
>>    fio --group_reporting --name=benchmark --filename=/dev/mapper/example \
>>        --ioengine=sync --invalidate=1 --numjobs=16 --rw=randwrite \
>>        --blocksize=4k --size=2G --time_based --runtime=30 --fdatasync=1
>>
>> results in an increase from 857 KB/s to 30.8 MB/s of the write
>> throughput (3580% increase).
>>
>> Signed-off-by: Yang Yang <yang.yang@...o.com>
> 
> I'm including my original fine-grained review comments inlined below
> BUT, having wasted time reviewing this patch I'm left with the
> conclusions:
> 
> 1) this patch has serious issues.
> 2) it fixes an issue with a toy 'example' but ignores that not all
>     targets are linear, each disparate target could have their own
>     reason(s) for actually _needing_ the redundant flushes.

I chose this specific 'example' because the constructed scenario closely
resembles real-world use cases.
This is a real-life scenario that we have encountered:
1) Call fallocate(file_fd, 0, 0, SZ_8G)
2) Call ioctl(file_fd, FS_IOC_FIEMAP, fiemap). In situations of severe
file system fragmentation, fiemap->fm_mapped_extents may exceed 1000.
3) Create a dm-linear device based on fiemap->fm_extents
4) Create a snapshot-cow device based on the dm-linear device

I guess targets needing the redundant flushes you mentioned are targets
with ti->num_flush_bios > 1. I think they won't be affected, duplicate
flush bios are still send to dm_dev by __send_duplicate_bios().
The main purpose of this patch is to ensure that __send_duplicate_bios()
is called only once for each dm_dev.

Anyway, I will modify this patch to apply only to dm-linear and dm-stripe.

> I'm inclined to never take this type of change.
> 
>> ---
>>   drivers/md/dm-core.h          |  1 +
>>   drivers/md/dm-table.c         |  7 +++++
>>   drivers/md/dm.c               | 59 +++++++++++++++++++++++++++++++++++
>>   include/linux/device-mapper.h |  6 ++++
>>   4 files changed, 73 insertions(+)
>>
>> diff --git a/drivers/md/dm-core.h b/drivers/md/dm-core.h
>> index e6757a30dcca..7e3f2168289f 100644
>> --- a/drivers/md/dm-core.h
>> +++ b/drivers/md/dm-core.h
>> @@ -217,6 +217,7 @@ struct dm_table {
>>   	/* a list of devices used by this table */
>>   	struct list_head devices;
>>   	struct rw_semaphore devices_lock;
>> +	unsigned short num_devices;
>>   
>>   	/* events get handed up using this callback */
>>   	void (*event_fn)(void *data);
>> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
>> index 41f1d731ae5a..ddc60e498afb 100644
>> --- a/drivers/md/dm-table.c
>> +++ b/drivers/md/dm-table.c
>> @@ -2133,6 +2133,8 @@ void dm_table_postsuspend_targets(struct dm_table *t)
>>   
>>   int dm_table_resume_targets(struct dm_table *t)
>>   {
>> +	struct list_head *devices = dm_table_get_devices(t);
>> +	struct dm_dev_internal *dd;
>>   	unsigned int i;
>>   	int r = 0;
>>   
>> @@ -2159,6 +2161,11 @@ int dm_table_resume_targets(struct dm_table *t)
>>   			ti->type->resume(ti);
>>   	}
>>   
>> +	t->num_devices = 0;
>> +
>> +	list_for_each_entry(dd, devices, list)
>> +		dd->dm_dev->index = ++(t->num_devices);
>> +
>>   	return 0;
>>   }
>>   
>> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
>> index 56aa2a8b9d71..7297235291f6 100644
>> --- a/drivers/md/dm.c
>> +++ b/drivers/md/dm.c
>> @@ -48,6 +48,8 @@
>>    */
>>   #define REQ_DM_POLL_LIST	REQ_DRV
>>   
>> +#define DM_MAX_TABLE_DEVICES	1024
>> +
> 
> This name is too general. This limit isn't imposed for anything other
> than bounding the size of the bitmap used to avoid redundant flushes.
> 
> So maybe rename to: DM_MAX_REDUNDANT_FLUSH_BITMAP_DEVICES

Got it!

>>   static const char *_name = DM_NAME;
>>   
>>   static unsigned int major;
>> @@ -1543,10 +1545,38 @@ static unsigned int __send_duplicate_bios(struct clone_info *ci, struct dm_targe
>>   	return ret;
>>   }
>>   
>> +static inline bool has_redundant_flush(struct dm_table *t,
>> +		unsigned long **bitmap)
>> +{
>> +	if (t->num_devices < t->num_targets) {
>> +		/* Add a limit here to prevent excessive memory usage for bitmaps */
>> +		if (t->num_devices >= DM_MAX_TABLE_DEVICES)
>> +			return false;
> 
> OK, in practice I'm not aware of tables that require such an excessive
> amount of devices.

Noted!

>> +		/* dm_dev's index starts from 1, so need plus 1 here */
>> +		*bitmap = bitmap_zalloc(t->num_devices + 1, GFP_KERNEL);
>> +		if (*bitmap)
>> +			return true;
>> +	}
> 
> This method is being used in the IO path, so you cannot use
> GFP_KERNEL.  Please switch to GFP_NOIO.

Got it!

>> +
>> +	return false;
>> +}
>> +
>> +static int dm_get_dev_index(struct dm_target *ti, struct dm_dev *dev,
>> +				     sector_t start, sector_t len, void *data)
>> +{
>> +	unsigned short *index = data;
>> +	*index = dev->index;
>> +	return 0;
>> +}
>> +
>>   static void __send_empty_flush(struct clone_info *ci)
>>   {
>>   	struct dm_table *t = ci->map;
>>   	struct bio flush_bio;
>> +	unsigned long *handled_map;
> 
> Please rename, e.g.: handled_devices_bitmap

Got it!

>> +	unsigned int nr_handled = 0;
>> +	bool check = has_redundant_flush(t, &handled_map);
>>   
>>   	/*
>>   	 * Use an on-stack bio for this, it's safe since we don't
>> @@ -1562,17 +1592,46 @@ static void __send_empty_flush(struct clone_info *ci)
>>   
>>   	for (unsigned int i = 0; i < t->num_targets; i++) {
>>   		unsigned int bios;
>> +		unsigned short index = 0;
>>   		struct dm_target *ti = dm_table_get_target(t, i);
>>   
>>   		if (unlikely(ti->num_flush_bios == 0))
>>   			continue;
>>   
>> +		/*
>> +		 * If the num_targets is greater than the number of block devices
>> +		 * in the dm_table's devices list, __send_empty_flush() might
>> +		 * invoke __send_duplicate_bios() multiple times for the same
>> +		 * block device. This could lead to a substantial decrease in
>> +		 * performance when num_targets significantly exceeds the number
>> +		 * of block devices.
>> +		 * Ensure that __send_duplicate_bios() is only called once for
>> +		 * each block device.
>> +		 */
>> +		if (check) {
>> +			if (nr_handled == t->num_devices)
>> +				break;
>> +
>> +			if (ti->type->iterate_devices)
>> +				ti->type->iterate_devices(ti, dm_get_dev_index, &index);
> 
> You're looping through all data devices in a target, so you're only
> getting the last device in the target's index.  That seems very
> broken.
> 
> But each target in your test 'example' device (mentioned in the patch
> header) only has a single device so you wouldn't have noticed this.

Perhaps the code comments are not sufficiently detailed, so allow me to
provide a detailed explanation of the execution flow of this code.

For device table only has a single device:
example: 0 8000 linear 8:2 0            -> dm_dev(8:2).index = 1
example: 8000 8000 linear 8:2 16384     -> dm_dev(8:2).index = 1
example: 16000 8000 linear 8:2 32768    -> dm_dev(8:2).index = 1
example: 24000 8000 linear 8:2 49152    -> dm_dev(8:2).index = 1
example: 32000 8000 linear 8:2 65536    -> dm_dev(8:2).index = 1
example: 40000 8000 linear 8:2 81920    -> dm_dev(8:2).index = 1
example: 48000 8000 linear 8:2 98304    -> dm_dev(8:2).index = 1
example: 56000 8000 linear 8:2 114688   -> dm_dev(8:2).index = 1
example: 64000 8000 linear 8:2 131072   -> dm_dev(8:2).index = 1

Before enter the loop:
     nr_handled = 0
     t->num_devices = 1

When i equals to 0:
for (unsigned int i = 0; i < t->num_targets; i++) {
     if (nr_handled == t->num_devices) //nr_handled=0, t->num_devices=1
         break;

     if (ti->type->iterate_devices)
         ti->type->iterate_devices(ti, dm_get_dev_index, &index) //index is set to 1

     if (index > 0) {
         if (__test_and_set_bit(index, handled_map)) //__test_and_set_bit() return 0
             continue;
         else
             nr_handled++;    //nr_handled = 1
     }

     bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
                      NULL, GFP_NOWAIT);  //__send_duplicate_bios() is called
}

When i equals to 1:
for (unsigned int i = 0; i < t->num_targets; i++) {
     if (nr_handled == t->num_devices) //nr_handled=1, t->num_devices=1, break the loop
         break;
}

__send_duplicate_bios() is called for device (8:2) only once.

======================================================================

For device table has two devices:
example: 0 8000 linear 8:2 0            -> dm_dev(8:2).index = 1
example: 8000 8000 linear 8:2 16384     -> dm_dev(8:2).index = 1
example: 16000 8000 linear 8:2 32768    -> dm_dev(8:2).index = 1
example: 24000 8000 linear 8:2 49152    -> dm_dev(8:2).index = 1
example: 32000 8000 linear 8:2 65536    -> dm_dev(8:2).index = 1
example: 40000 8000 linear 8:3 81920    -> dm_dev(8:3).index = 2
example: 48000 8000 linear 8:3 98304    -> dm_dev(8:3).index = 2
example: 56000 8000 linear 8:3 114688   -> dm_dev(8:3).index = 2
example: 64000 8000 linear 8:3 131072   -> dm_dev(8:3).index = 2

Before enter the loop:
     nr_handled = 0
     t->num_devices = 2

When i equals to 0:
for (unsigned int i = 0; i < t->num_targets; i++) {
     if (nr_handled == t->num_devices) //nr_handled=0, t->num_devices=2
         break;

     if (ti->type->iterate_devices)
         ti->type->iterate_devices(ti, dm_get_dev_index, &index) //index is set to 1

     if (index > 0) {
         if (__test_and_set_bit(index, handled_map)) //__test_and_set_bit() return 0
             continue;
         else
             nr_handled++;    //nr_handled = 1
     }

     bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
                      NULL, GFP_NOWAIT);    //__send_duplicate_bios() is called
}

When i equals to 1, 2, 3, or 4:
for (unsigned int i = 0; i < t->num_targets; i++) {
     if (nr_handled == t->num_devices) //nr_handled=1, t->num_devices=2
         break;

     if (ti->type->iterate_devices)
         ti->type->iterate_devices(ti, dm_get_dev_index, &index) //index is set to 1

     if (index > 0) {
         if (__test_and_set_bit(index, handled_map)) //__test_and_set_bit() return 1
             continue;
         else
             nr_handled++;
     }
}

When i equals to 5:
for (unsigned int i = 0; i < t->num_targets; i++) {
     if (nr_handled == t->num_devices) //nr_handled=1, t->num_devices=2
         break;

     if (ti->type->iterate_devices)
         ti->type->iterate_devices(ti, dm_get_dev_index, &index) //index is set to 2

     if (index > 0) {
         if (__test_and_set_bit(index, handled_map)) //__test_and_set_bit() return 0
             continue;
         else
             nr_handled++;    //nr_handled = 2
     }

     bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
                      NULL, GFP_NOWAIT); //__send_duplicate_bios() is called for the second time
}

When i equals to 6:
for (unsigned int i = 0; i < t->num_targets; i++) {
     if (nr_handled == t->num_devices) //nr_handled=2, t->num_devices=2, break the loop
         break;
}

__send_duplicate_bios() is called only once for each of devices (8:2) and (8:3).

>> +
>> +			if (index > 0) {
>> +				if (__test_and_set_bit(index, handled_map))
>> +					continue;
>> +				else
>> +					nr_handled++;
> 
> Think you really mean to set bits in the bitmap within the
> iterate_devices callout.  So it should be renamed accordingly.
> 
> Why not count the first time a device is handled in nr_handled?
> Also, it strikes me as strange that you're break'ing out this loop
> early based on nr_handled... not seeing the point (and also it seems
> broken, because it implies you aren't issuing flushes to targets at
> the end).
> 
>> +			}
>> +		}
>> +
>>   		atomic_add(ti->num_flush_bios, &ci->io->io_count);
>>   		bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
>>   					     NULL, GFP_NOWAIT);
>>   		atomic_sub(ti->num_flush_bios - bios, &ci->io->io_count);
>>   	}
>>   
>> +	if (check)
>> +		bitmap_free(handled_map);
>> +
>>   	/*
>>   	 * alloc_io() takes one extra reference for submission, so the
>>   	 * reference won't reach 0 without the following subtraction
>> diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
>> index 82b2195efaca..4a54b4f0a609 100644
>> --- a/include/linux/device-mapper.h
>> +++ b/include/linux/device-mapper.h
>> @@ -169,6 +169,12 @@ struct dm_dev {
>>   	struct dax_device *dax_dev;
>>   	blk_mode_t mode;
>>   	char name[16];
>> +
>> +	/*
>> +	 * sequential number for each dm_dev in dm_table's devices list,
>> +	 * start from 1
>> +	 */
>> +	unsigned short index;
> 
> Please update this comment to indicate that index=0 is (ab)used as a
> flag in __send_empty_flush().

Noted!

Thanks for your comments.

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